I'm having the same issues on the same machine (MacBook Pro 2015, Maximum Everything Hardware-wise). After the room floods, I can even load the next level where I'm marching around underwater, but the graphical glitches while submerged in "water" are pretty remarkable... Objects (like plants/seaweed) seem to flicker in and out of existence. Brightly colored, organic/crazy looking objects appear out of nowhere, often obscuring the field of view until they are physically passed through. The game basically becomes unplayable.
I found this thread that seems to address the particular issue I am seeing:
http://www.frictionalgames.com/forum/thread-30680.html
For the time being, there seems to be a manual workaround.