Getting into a ceiling looks like it's all about positioning as soon as you find one that's compatible:
In theory it should be possible in every location like the above (this is in the first swimming section, around frame 3400 in the submission), though for an optimized TAS I would check every single ceiling along the route even if it doesn't look like you'd be able to get in. It might not work immediately: You'll probably have to adjust when you press A to swim upward alongside holding Up or Down to further fine tune the position. Once you're in the ceiling proper, you'll want to hold Up the entire time, and find the frame where pressing A zips you forward. If you can't find that frame, go back to adjusting your position: There may be a frame where you get zipped downward out of the ceiling entirely, repositioning a bit can fix that. Once you can zip, A can be pressed every 4 frames from there to continue zipping at top speed,
it should look like this in TAS Editor when done optimally (hyperlinked to image because it's a bit tall).
I've got so many other projects at the moment on top of the December submission rush, otherwise I'd look deeper into both that and the floor clip, but hopefully that should be a good start for an improvement! oops i broke it
User movie #68199453342958203
Decided to look into the floor clip. I held Down + Left until going as far down as I could. Holding Right from there and then tapping A a few times will complete the clip and put you under the level... Which is essentially a giant ceiling. Zips all day. That userfile is a 1000 frame improvement by itself and it likely could be improved further, I was admittedly rushing it just to get the knowledge back to you. The slowdowns are to avoid swimming back up into the level (or into weirdly placed "holes" in the floor ceiling, that sure was a set of words I just typed and meant). Turns out you might not need to hold Up for ceiling zipping, though I'd imagine it never hurts to do it anyway.