At sporting goods or fishing stores, they have submersible 12V pumps that are fairly small for aerating bait tanks. Most have a set of alligator clips that you can just cut off. I can't recall what the pump part of the assembly looked like. I have no idea how deep they can be submersed though.
If you manage to bypass/remove the regulator circuit inside 12V Cassette player motor, it won't have any problem running inside water. I had tried it long back when I was doing a school science project and it works.