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.
Using pause buffering, it is possible to gain speed via superswimming by alternating between Up and Down on the control stick between every pause. However, once you gain ~550 units of speed, you can actually just hold Up to continue charging a superswim due to how the games target camera (the default camera when the freecam is off) works. Once you have enough speed, you can release the swim in whichever direction you're facing.
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 determines the speed he has. 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 superswim 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 when releasing at around two-thirds of the total charging time, you can still get low 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 backward at 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 on out of the 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 onto land, your speed is retained, and Link will run backward. 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. This allows Link 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.