The Swift Sail is an upgraded version of the Sail and a new item added in the HD re-release of The Legend of Zelda: The Wind Waker. The Swift Sail allows Link to sail at greater speeds than the Sail allows; it also changes the wind direction depending on the direction in which he is sailing, removing the need to play the "Wind's Requiem" to sail in a different direction.
Link can purchase this item when it randomly appears in the Windfall Auction.
The Swift Sail appears as the Daphnes Nohansen Hyrule's Level 2 weapon in his Sail moveset. It is unlocked by obtaining its Sealed Weapon from a Treasure Chest in Wind Waker: Watchers of the Triforce.