I was making an enclosure made of glass trapdoors from Upgrade Aquatic for platypuses to live in and filled it with water,seagrass and a little sand beach section so they could get up and breathe. But some of the platypuses dont go up for air and end up drowning.

My Enclosure

I've done some experimenting and it appears that when platypuses are in water contained by any trapdoor they attempt to breathe from the trapdoors rather than go up to surface. I assume this is because of how trapdoors are "half" blocks and how mobs can clip through slightly and thus assume that the trap door is the quickest route to get air