World of Illusion swimming

SWIMMING

I also figured out the swimming.
As we already knew, there are different swimming speeds.  There are actually a ton of different swimming speeds, not just 4 (but many more than 4, maybe thousands, although the game likely rounds them somehow).  Also, as we suspected holding right as the level starts results in Mickey getting maximum speed.

As long as you're holding right and and Mickey is not touching the ground, the swim speed will never change, even if you hit a wall and stop, once you start moving right again you'll have the same speed.
While Mickey is swimming forward on the ground, the speed oscillates like it does when running on ground (every frame, albeit with different values).  Once you leave the ground, you'll be stuck with whatever speed you were at (same as jumping on land).

As I said above, if you're off the ground and holding right your speed will remain constant.  If you hit left then right again your speed will likely end up with a new value. I suspect that this depends on how many frames you aren't hitting left or right (since going from right -> left -> right won't be frame perfect).  Your speed will slightly decrease for each one of those frames (probably not enough where you'd notice).  I'm not positive on how this works , but it doesn't really matter (see below).

There are 2 parts in the level where you must hit left and/or touch the ground.  The first is during the vertical shaft, and the second is when waiting on the clam.  I was able to discover how to achieve maximum speed when you lose it.  The trick is to get Mickey's speed to 0 while off the ground, then hold right.  This results in the maximum speed (and that's how you get maximum speed when the level loads, since you start with a speed of 0).

For the vertical shaft, as I'm going up at the end I make sure my horizontal speed is 0 (it will naturally go to 0 if you don't press left or right), then just hold right after clearing the top.  For the clam, wait on the ground until it covers the bubble, jump (to get off the ground), then press right.  I guess you could also wait against the bubbles off of the ground, that should work as well since your speed will be 0 while waiting.

Random finding:
The difference between the maximum swim speed and the slowest swim speed equals the difference between speed 4 and speed 1 on the ground.