The main problem in thruster design is how to isolate engine from water. There are three solutions used to keep engines away from water:

1. O-ring
2. Rotary shaft seal
3. Magnetic coupling

The best and more complicated is solution 3. The engine is perfectly isolated from water because it is connected to the propeller using magnetic coupler. Solutions 2 and 3 are similar. In addition it is recommended to fill the engine housing with oil to compensate pressure.

I plan to build custom made brushless engines that give me magnetic coupler level of isolation.

Magnetic coulper

Schematic thruster view

Two isolated parts of thruster.

Animated thruster parts.

Click above picture to download animation (WMV9).