When attempting to swim through a monument (or near any objects underwater, but the monument makes it very noticeable) your character can glitch in and out of the swimming animation. The camera jerks around when it does it, and speed is dramatically effected. I've noticed it happens the most when attempting to swim into a space with only 1 block of vertical clearance, or when swimming near the bottom of any blocks. Swimming seems fine when you aren't near any blocks. 

I currently do not have a gif of the issue, but I can put something together if requested.