I was thinking about a hydraulic pump too. Just like the ones used in heavy machinery, but smaller and plastic. Basically it's a pair of gears meshing together pulling water around the edge of the gears from the side where the teeth move apart to the side where they come together. I'd like to try building one. Remember, electric motors work underwater
No need to seal driveshafts going through pump housings.
well, it depends on motor type.. you need the kind with the wire coil on the stationary part, mags on the shaft.