I've been trying for a while and I finally got it, then do something completely wrong. Instead of using the Wind Waker, I talked to the boat. I don't really remember what happens when you do that but I do't want to spend all day to try and do a Super Swim for 30 seconds. Help Me.
Getting (Dry) Storage
- Climb a ledge that you can easily fall off of similar to the one in the video.
- As soon as you climb the ledge take out the wind waker.
- As you fall, put away the Wind Waker (press B) 3 frames before you land. This will try to cancel the "cutscene" twice, so it will essentially cancel the next "cutscene" you enter (opening chests, taking out the wind waker again, etc.). This will activate dry storage.
You know you have dry storage when you have the Wind Waker out (Link is holding it) but it doesn't show the diamonds or metronome thing.
With Dry storage activated, hold Lock-on, jump into the water, let go of Lock-on, and hold up on the control stick. You'll gain speed exponentially and once you feel you are at a high enough speed, mash Up on the D-pad while still holding up on the analog to open the sea chart to see your position. Once the area you want to head to is opposite of your direction, let go of up on the analog stick and close the sea chart.
A super swim can also be done without dry storage by mashing the start button (frame buffering) and changing the direction of the control stick every frame. Not recommended cause it takes forever to get to a decent speed.
The frame-buffering method (above) is probably the easiest, but it does take a loooooong time. Look how long people are twitching around, usually 10 seconds or so, then multiply that by the frame rate (30 fps) and that's how many inputs you need to pause-buffer: 300!!! Pause-unpause takes about 2 seconds, so that means it's about 10 minutes of repetition (and if you screw up you're at square one). Spend an hour or so practicing dry storage and you'll probably enjoy it more.
It's also do-able with TAS, but that's generally considered a separate category if you're trying to speed-run.
For something more in-depth, here's a longer tutorial video: