Pressing alternate directions on every other frame while swimming will cause Link's speed to increase linearly at 3 units of speed per frame. if charged up for long enough, this can allow Link to travel between islands of the great sea without needing the sail or the King of Red Lions.
Although the general trend while charging is a major increase in speed over time, Link's speed while charging does not constantly increase. Link's swimming animation occurs in a cycle, and which frame he is at in the cycle will determine what speed he will get. As a result, Link's speed will drastically fluctuate depending on which frame in the swimming animation's cycle he is currently in.
There are several ways to make superswims speed and distance more consistent in a speedrun. After releasing a superswim, you will lose two units of speed every frame. If you release too early, you will not have enough speed and will not travel far enough. If you charge for too long, you will not have enough air and will drown too soon. With this in mind, the optimal time to release a superswim for maximum distance is about two-thirds of the total charging time. However, even if releasing at around two-thirds of the total charging time, you can still get bad speed and not travel far enough.
The following graph illustrates the correlation of frames and overall velocity for one air gauge worth of time:
Once 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 looking at your map.
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. With enough speed, it is possible to reach an island before it loads. Using this allows to be loaded underneath an island, similar to a Wind Waker dive.
Superswimming can be used to cross vast amounts of overworld extremely quickly (much faster than sailing). The sail is still a necessary item, however, as not obtaining it will cause softlocks later in the game.