This part will take you through the entire Lakebed Temple.
Inside the Lakebed Temple
Entering the temple
In the tunnel where you enter the temple, swim carefully through it to avoid the jellyfish-like Baris that will shock you and the clam-like Shell Blades that will snap at you. When you reach the first room of the temple, you'll see a door that you can open by jumping off a ledge toward a pull hook. Be careful of the gelatinous Chus that are in this room. Slay them, and they will leave behind puddles of Chu Jelly that you can collect with your bottle if you're fast enough to scoop up some. What you want to scoop up is the Red Chu Jelly, which will restore eight hearts of health to your meter, and Blue Chu Jelly, which restores all your hearts. Purple Chu Jelly has random effects, so avoid collecting that, while Yellow Chu Jelly acts like lantern oil, which you don't need in here.
In the next room there is an incline that will take you down to the bottom of a series of ledges we must climb up to get to the next door. To get up on some of the ledges, though, we're going to need to blow up some rocks that are hanging on the ceiling, so equip yourself with the arrow-and-bomb combo and aim for those rocks to blast them down. On the ledges are some small creatures called Helmasaurs that wear protective shells, which will make frontal assaults useless. We don't have a weapon yet that can remove the shells, so the only way to destroy them at this point is to rotate around them when they charge at you so that you'll be behind them, then give them a jump strike with your sword. But given where they appear in this room, your best bet is simply to avoid them. Behind the door is a bridge we need to cross to get to the next room, and on that bridge is a Lizalfos that will charge at you, which you can easily take down.
The western water gate
The next room we enter is a two-floored chamber with a staircase that rotates around to various sections of the chamber when we jump onto and grab the pull hooks that hang near the ledges. This acts as a hub for the entire temple, because we'll be going through this area many times.
First off, we want to go down the stairs to the bottom floor, then go to the right, slaying Tektites that get in your way, until you are at the far south of the room. Jump toward the pull hook there and grab it to rotate the stairs, then go up to the top floor and go to your left until you see some jars. Smash them, and one will release Ooccoo who will act as your transport out of the dungeon at any time should you need to leave. Now go to the right to get to the east end of the room and grab the pull hook there to rotate the stairs so you can go down to the bottom floor again. Go to the right to open the treasure chest that will give you the Dungeon Map, then open the nearby door and head west across the bridge, avoiding or defeating the Helmasaur in that location.
In the room beyond the bridge we see three platforms hanging on chains and some water geysers near the floor. Use a bomb arrow to knock down some rocks hanging over the geysers, then go through the passageway on your right to travel down toward a pillar with a vine we can climb across on to get to a ledge next to some platforms leading to a treasure chest. Time your jump across the platforms when the geyser with the rock on it reaches its full height, then open the chest to get the small key. Go back to the door the same way you came (if you fall down, there are some ledges you can climb up on to get to the one near the door), and go across the bridge to reenter the central hub room.
From there, go back up the stairs to the top floor and travel to the left until you reach the locked door. Open it and cross the bridge, avoiding or defeating the Helmasaur patrolling it, until you reach a hallway with a gutter that goes to the left. Defeat the Tektite, then go to the right where there's a gate and some rocks hanging from the ceiling to blast down with some bomb arrows. Get on the rock near the vines to climb up on the wall and then above the doorway where there's a pull hook to grab, lowering the gate. Drop down and continue down the hallway to the west until you come across a big boulder to blow away with a bomb arrow. Save the boulder for later and go through the door that leads you into a room with a giant cog for a platform in the center. Try not to fall off it as you cross over to the door on the opposite side of the room, slaying the Lizalfos that will charge at you, then go through the door and get the small key from a treasure chest in the nearby hallway. Now go back through the room into the hallway to blow up the boulder. Behind the boulder is a small passageway with some Helmasaurs to dodge out of the way as you head toward a room where an insect-like creature called a Chu Worm jumps inside a gelatinous bubble to attack you. Use a bomb arrow to blow away the bubble so you can easily dispatch the Chu Worm inside.
Unlock the door and you'll enter a chamber with a spiral walkway going around its outer walls, and a pull hook near a platform in the center. Turn to the right and jump off the ledge, then climb up the vine to get to the walkway and just go straight on that path, slaying the Tektites that get in your way. When you reach the archway at the top, climb up the ladder until you are on top of the archway, then jump toward and grab the pull hook. This will now release water that will go down the spiral walkway. Climb down the ladder and then just slide all the way to the bottom until you are able to swim to the platform in the center of the chamber from where you can jump and grab onto the pull hook there. This will open up a gate that will cause water to flow through the gutter into the central hub room.
Go back to the room with the giant cog, which you'll now see is turning with the flow of water, and drop down into the room below. You'll now see that the three platforms on chains are also moving. Get yourself close to one of the moving platforms so you can ride it and jump to the ledge where the door to the north is located. Go through that door, walk to the left in the next room to grab a small key from the treasure chest, then return to the room with the rotating platforms so you can ride one to the ledge with the door to the west. In the next room immediately open the locked door that is straight ahead, then put on your Iron Boots and take a walk underwater through some strong currents. There are Baris to avoid and a Shell Blade to either kill when its mouth is open or just avoid by taking off the Iron Boots and swimming past it.
At the end of the underwater passageway, rise up to the surface and you'll enter a room where you'll need to look up to see a slimy creature hanging from its ceiling. This is the Deku Toad, and on its back are tiny little Toados he will send out to attack you. You can take a good deal of them out by using the spin attack. Once the swarm is gone, Deku Toad will rise up and then try to squash you flat by landing on top of you. Watch for its shadow and get out of its way before it lands flat on its stomach. The toad will be stunned, leaving his tongue hanging out for you to strike a few times with the sword. He will revive himself and then sprout some more Toados to attack you. Follow the same method of attack two more times to defeat him. Once he's down for the full count, he will expel a treasure chest that will give you your next useful item -- the Clawshot. This works like the Hookshot in other similar Legend of Zelda titles in that it will pull you toward certain stationary objects and it can pull other objects towards you, plus it can be used as an offensive weapon. The Clawshot also has an extendable chain that you can use to raise or lower yourself while hanging on the chain.
So on to exiting the mini-boss room: go over to where there's a target for you to use the Clawshot on and aim for it so you can reach the ledge. This will cause the gate on the ledge to open, so you can drop down and exit the room through the door.
The eastern water gate
Make your way back into the central hub room, where you'll see water flowing down into the circular pool below, causing the water level to rise. Along the way, you can try out the Clawshot on removing the shell of the Helmasaur so you can easily attack it from the front. From where you reenter, grab the pull hook in front of you to make the stairs rotate so you can go up to the top floor, then travel to the right until you're at the door where the water is flowing from, then aim at the Clawshot target so you can rotate the stairs again and cause the water to cascade down the stairs, giving you a short ride down. Go through the door and across the bridge underneath the giant waterwheel, slaying the Tektite that's in your way, to reach the door that will take you into a room with some more platforms on chains, only these aren't moving yet. Jump onto the nearest platform, but watch out for the Keese that are flying around. Look for a Clawshot target to appear on your left and aim for it, then descend down to a stone platform below you. You'll find a treasure chest that contains water bombs there. Now look for a patch of vines on a pillar that you can Clawshot your way toward, then climb all the way to the right on it until you reach a ledge. From there Clawshot onto another pillar with a patch of vines that's higher up, then climb all the way to the left until you reach a ledge. Then Clawshot onto some vines on a wall near a balcony and climb all the way to the right so you can drop down and then go through the door.
You'll enter another hallway with a gutter passing through it, and on your left will be a Chu Worm in a bubble you should dispose of with your Clawshot and your sword. Go to the right, and you'll see a rock on a ceiling hanging over a geyser. Shoot it down and then climb onto the rock when the geyser isn't gushing so you can jump onto a ledge above the doorway and then jump down on the other side. Slay the Tektites, then go over toward the door with the Clawshot target over it and pull yourself toward the door. In the next room there are two Chu Worms in bubbles to get rid of. Here we see that the gutter splits into a Y, with the other gutter heading through the door to the right of the one we went through.
There's another chamber up ahead with a spiral walkway going around its outer walls, and a pull hook near a platform in the center. However, there's no door to get into that chamber, but there are vines growing on the ceiling that we can Clawshot onto and then climb along until we're inside, making sure you land on the ledge to the left. Clawshot onto the vines growing on the other ledge, then climb up the vine to get to the walkway and just go straight on that path, either avoiding or defeating the Helmasaurs in your path. Partway up the path, though, there are two breaks where we need to Clawshot toward a target to continue along. To get past the first break, we'll have to watch out for the geyser and get ourselves across when it isn't erupting. When you reach the archway at the top, Clawshot into the alcove ahead of you to open up the treasure chest that contains the Compass. Then climb up the ladder to the top of the archway and jump forward to grab the pull hook. This will now release water that will go down the spiral walkway. Climb down the ladder and then just slide all the way to the bottom until you are able to swim to the platform in the center of the chamber from where you can jump and grab onto the pull hook there. This will open up a gate that will cause water to flow through the gutters into the central hub room.
Finding the big key
Go back to the room with the vines on the pillars, this time going back down one floor (yes, you can shoot down to the lower vines!) so you can hook onto the Clawshot targets underneath the giant cog near the door to the east and ride it until you reach that ledge. Go through that door and then take a dive into the water on your left, swimming below an underwater shelf. In a grotto to the north near the shelf you will see a boulder on your left blocking a hole. Here we're going to need a water bomb to blow up the boulder, but in a rather convenient sort of way, a Bomb Fish will show up and harass you in that spot. You can strike the Bombfish and use it in place of a bomb to blow up the boulder. Swim through the hole and head to the south where you'll see another boulder near the bottom to use a water bomb to blow up. Go through that passageway and then rise to the surface; you'll arrive next to a door. Go through it and take out the Chu Worm in the next room, then latch onto the Clawshot target on the ceiling. This will open up a hatch for Link to descend down into another room onto a platform where the treasure chest that has the Big Key is located. Once you get it, sink down into the pool and take out the Skullfish swimming in there with your sword before taking the seaweed-filled passageway out, watching for the Baris that float around in there.
From there, make your way back into the central hub room, where you'll see water flowing down into the circular pool below, causing the water level to rise again. Now jump into the water and swim towards the door in the center of the chamber that you can now open with the Big Key. Then drop down the hole behind the door to face the dungeon boss.
Lakebed Temple Boss: Morpheel
After Link falls into the hole, put on the Iron Boots to have him sink to the bottom of the dungeon chamber. When he reaches bottom, he will see a creature with tentacles emerge from the sand. This is the dungeon boss Morpheel, referred to also as Twilit Aquatic. In the first part of the battle, he will rely mostly on his tentacles to grab you and put you into his mouth to chew on you before spitting you out, causing some heart (meter) damage. You want to stay out of reach of the tentacles and aim your Clawshot at the eye that passes its way through the tentacles. Once you latch onto it and draw it out, strike at it with your sword. He will then send out some Bombfish to act as decoys for your Clawshot. Repeat the same method of attack on Morpheel's eye two more times, and then it will emerge from the sand and start swimming around.
Now begins the second part of the battle. Have Link remove his Iron Boots so that he can swim around and get close enough to Morpheel's eye, then latch onto it with the Clawshot to pull Link toward it so he can slash it with the sword a few times. Morpheel will then buck and shake Link off him. Again, swim after Morpheel and Clawshot onto his eye to strike at it two more times. With enough hits, Morpheel will then run into the wall and his body will harden while his eye drops off, which will turn into a Heart Container. Morpheel's body will then explode into Twilight bits which will form into the last Fused Shadow that Midna will collect after the water drains from the chamber.
You have completed the Lakebed Temple. Collect your Heart Container, and enter the portal to leave the temple. You will be asked to save your game at this point.