It's possible to walk underwater

I'm not sure how replicable it is, but I got the game to forget that I was underwater. Gravity and all the other mechanics work as normal, but it's impossible to get back out if you can't jump to the surface of the water.

https://imgur.com/a/3AkTAoK

This is probably not all that useful, but it could help if you're struggling with >!the underwater Headless fights!<. I imagine they'd be a lot easier to fight conventionally, for what that's worth. Hypothetically it might be useful for speedrunning, because the floor isn't meant to be walked on -- there's potentially some way to clip through the ground here.

I managed to do it by swimming up with L2 then letting go the frame before you would enter the surfacing animation. You'll be hovering in midair with the falling animation for a few seconds, then drop to the bottom.