Superswim
Discovered by Klydestorm
Perform Storage, and Camera Lock with the Wind Waker. Get into a body of water, and hold Up on the analog stick. Link will begin to spin around rapidly, gaining speed with each turn. When you're satisfied with your speed, release the analog stick with Link's back pointing where you want to go, and Link will rocket backwards with the speed you attained. You can better judge where you're aiming by opening up your map.
Pressing alternate directions on every other frame while swimming will also cause Link's speed to increase exponentially. This is barely possible on console using lots of pauses, however, and will likely never be worth doing. Using Forest Water storage gives you this same effect due to the way the camera works while the glitch is active.
The only limit to the speed you can gain is your air meter, and the positioning of the land, including things you can climb up on out of water. You can continuously refresh your air meter if you happen to swim close to the shore of an island without going on land. If you Superswim up on land, your speed is retained, and Link will run backwards. If you go off an edge or back into the water while on land, your speed will reset back to 0.
A TAS would use superswimming to cross vast amounts of overworld extremely quickly (much faster than sailing). The sail is still a necessary item, however.