Just throwing out some other off the wall propulsion ideas. The DC brushless approach is by far the best even if they end up having some kind of replacement cycle due to wear.
Submersible pond and fish tank pumps are already made potted. They have the major draw backs of probably using too much current and producing too little torque, especially with the larger impeller that would be need to be mounted. They also need AC current, but you are already headed that direction. I suppose with some ability to change the AC frequency, you could get more push from such an induction motor.
Depending on what kind of wave patterns, frequencies, voltage range, and power an I/O board like Arduino could put out, one might be able to fashion something similar to the so-called Dyson Digital motor using off the shelf air core inductors placed on an iron core armature and potted / resin filled throughout. The motor is essentially a half-breed between AC induction and DC brushless.
A hydrostatic drive coupling / torque converter would probably be easier to use, but the smallest ones I can think of are for riding lawnmowers. Similarly, a pneumatic or hydraulic drive system would add unnecessary complication.
Last crazy idea. Voice coil and armature from a computer HDD. Proper frequency input and a fin and it can swim like a fish. Actually, any sealed voice coil has interesting possibilities. Why yes, that is the sound it makes at full speed.
By the way, are you potting the excess space inside the DC motor casings? That would add some level or corrosion resistance in case the winding insulation reacts poorly to salt water as well as insulating solder junctions. I know I heard something about an added water proofing coating, but wasn't sure what procedure you are going with.