When holding down W to swim and hitting something, the player actually stands up but the client immediately makes the player swim again, thus causing a disagreement between client and server, and therefore a desync (you can barely notice that in the attached video so I recommend testing it out on your own since its a lot more obvious if you do it yourself). The server thinks the player's hitbox is standing upright but the client thinks the hitbox is still the size when swimming.

Steps to reproduce:
1. Swim and continue holding W
2. Punch something without releasing W
Results: You stand up for a short while and then swims again, causing a desync. You cannot swim through 1 block gaps and cannot touch the ceiling.

Video demonstrating the issue: https://youtu.be/V42Vezw24Sw