Waterproofing motors is tricky. Very tricky. That's why it's great that someone else has already done it! Bilge pumps are used to remove unwanted water that collects in the bottom of the hull (the bilge) of a boat. Because they spend a lot of time submerged in water, bilge pumps are made to be waterproof. By cutting off the outside housing of a bilge pump and removing it's impeller, you can gain access to the shaft coming from its 12v motor through a waterproof fitting called a stuffing box. A special adapter can be made to attach a propeller used in electric ducted fan-powered model planes, and "voila!" you've got yourself a waterproof propulsion system!
Figure 1: Electric ducted fan propeller attached to a hacked Rule 360 bilge pump motor