Well my fire does it too and he's one of the pains in the ass who drops his rats into the water bowl sometimes. So have fun with him too lol.
I can't do the pvc idea because i use newspaper as substrate.
I usually only fill them halfway. I'm telling you, i've watched them do it even with the ceramic bowls. They push on the front of the tub with their face and the water bowl with their neck at the same and tip them so water spills out. A few times i've seen them do this and then go right back into their hides. They aren't out actively hunting or have a messy tub, they just do it for fun it seems...