TORQUE REACTION ENGINE: 

Motors change acceleration of a symetric flywheel, typically formed from the Phish's batteries, where the flywheel, motors, and control system are within an isolation chamber. 

I call this a "torque reaction engine" or "TRE". When the motors change acceleration of the inertial mass of the flywheel, this causes an opposing torque reaction on the motors, which is communicated to the isolation chamber.  A fin is attached to the isolation chamber. Moving the fin through surrounding thrust fluid generates lift/thrust and drag. 

Because the system is symmetric and there is essentially one powered moving part, the system has excellent scaling factors, with efficiency and deployment time increasing with size. It may be possible to build Phish as large as 100'. 

Because there is no penetration in the isolation chamber, the system will be capable of EXTREME depth (+6000 meters). Torpedoes and other subsea watercraft with propellers are limited in depth by leakage up the driveshaft. This can be controlled, but always with effects on efficiency.