https://gfycat.com/TalkativeSatisfiedInvisiblerail

As a quick preface, this glitch currently does not have any use in speedrunning and likely never will, but it would be nice to keep documented in some way.

General info

We've known for a long time that Yoshi's swallow animation has a strange behavior where state changes don't always work properly during swallow. This is most notable in the JP version, where you can jump midair while swallowing if you slide off an edge while eating a fruit.

It turns out this also applies while swimming, where Yoshi cannot transition from swimming to walking while he is swallowing a fruit. This means you can use a slope to nudge Yoshi above the water line during his swallow animation, and the game will put him back into the swimming state, because none of the conditions to leave the swimming state have been triggered. (The "swimming swallow" state does not have any conditions to leave the swimming state.)

Entering the loading zone while Yoshi is swimming simply puts Yoshi on the ground again.

How to control air swimming

The swimming state itself checks regularly to see if Yoshi is transitioning from water to ground. Here is how the checks work.

Swimming upwards: Ground detection never happens. You can swim upward infinitely and soar into the skies. Swimming horizontal left/right: Ground detection happens every stroke when the animation loops. Yoshi will exit swim and start to fall at the end of the stroke. By interrupting the stroke animation with tongue, you can avoid this. Example: https://gfycat.com/ConcernedElasticElephantseal Swimming downwards: Ground detection happens every frame. Yoshi will exit swim and start to fall immediately.

Where is this possible?

The glitch is only possible in levels that have a slope entering the water, and something to eat next to the slope. I believe this is only 4-2.

Special thanks to LoveBot for accidentally discovering this glitch.