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

• O-ring
• Rotary shaft seal
• 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 was planing to build custom made brushless engines, but finally decided to build thruster with magnetic coupler.

Magnetic coupler

Schematic thruster view

Two isolated parts of thruster.