I don't have footage of it, so here are the instructions:

You need to sink into the water with inventory drop. After you start sinking, you need to cast a spell. I like using D-Dark. Right after you do the spell, tap left or right to cancel out of it. Since you didn't let the Knight to sink very far, you can jump out of the water holding your jump button. The Knight will have some shining ball in his face and he will start ascending.

Sounds like dive float. Typically this is done with quick map storage (so that it can be done anywhere, not requiring a pool of water). Nice job finding it on your own though!

Thanks a lot! I thought it would be possible with map storage, like the other actual new glitch I found where I left the instructions here: https://www.speedrun.com/hollowknight/forums/h4o4y

Maybe that glitch will be helpful some time. Thanks for responding!