Sail

From Zelda Wiki.org

The Sail is an extremely important item in The Wind Waker, obtained early in the game. The sail is the item which allows Link to catch prevailing winds, controlled by The Wind Waker, to sail from island to island. The sail can be set to either the X, Y or Z button on the GameCube controller, and can only be used while onboard the King of Red Lions.

The Sail is purchased from Zunari on Windfall Island for the reasonably small price of 80 Rupees.