Now that we have the sacred crest imprinted on the Southwestern Sea chart, head back down the path to Mercay Island Harbor and meet Linebeck at the boat to tell him. Since this crest appears in the middle of the sea, it's apparent that we're going to need something for the boat in order to get what's in that location. Right around this time you should be receiving a message from Eddo telling you that he's got a Salvage Arm ready to install. If not, a man on the dock will shout at you, telling you you're going to need a Salvage Arm installed on your boat. So let's ship off to Cannon Island again, watching out for the same hazards as before on the sea.
Once you're back at Cannon Island, you can go into Eddo's Garage by the door that he's now unlocked and talk to him about getting the Salvage Arm. However, Eddo will now judge how much you'll give him for that part by how loudly you shout, so shout loud into the microphone (best to do it in privacy, though!). If your shout is loud enough, he'll give you the Salvage Arm for as low as 200 rupees; otherwise, he'll ask for 1000 rupees if it isn't loud enough. If that's the case, then leave the workshop and come back to try again.
Getting the Sun Key
Now that we have the Salvage Arm for the ship, climb back on board and plot a course to where the sacred crest is on the map. Once we get there, it's time to activate it and start dredging the sea for the treasure we're looking for.
On how to operate the Salvage Arm: use the slider at the bottom of the touch screen to move the claw left and right and to adjust your descent and ascent speed. What you want to watch out for are rocks and Octomines, since these can inflict one heart damage to the Salvage Arm upon contact. If you lose all your hearts, the Salvage Arm is rendered inoperable, and you'll have to take it back to Eddo for repairs, which will cost you rupees.
So on your descent here, you'll come across four Octomines in a diamond pattern, with a rupee inside a bubble in the middle that you can snag for cash. Then in a rocky chasm there will be three Octomines that will travel back and forth, left and right. This will be followed by four more Octomines holding position in a tighter diamond pattern around a rupee in a bubble. Avoid the rock that appears on the left, and to its right is the treasure chest. You'll see "CATCH!" appear on the screen when the claw latches onto the chest. Now slowly and carefully pull it back up to the surface while avoiding the same obstacles you have passed by on your way down, making sure that they don't hit the chest or the claw.
Once you get the chest aboard the ship, Link will open it up. It contains the Sun Key, which Linebeck will wonder what we're going to do with it. Since we remembered a sun-marked door that was locked in the second hideout of Romanos' father on Molida Island, it's time for us to return to that island and unlock the door. So set a course for Molida Island.