I am working with a friend of mine to create an underwater propulsion system. My concept is to use an open hydraulic system to allow human generated torque to be converted to water pressure then back into torque to spin a propeller. It my seem counter intuitive, but I actually allows me great flexibility in locating the propeller that I wouldn't otherwise have. Horsepower and RPM are minimal at <2 and 90 respectively.

My question is what type of pump and motor would you recommend to use? True water based hydraulics offer a level of precision, pressure, and torque (not to mention cost) I just don't need. Is there a type of off the shelf pump that could be run backward efficiently as a motor? Eventually I need to build a proof of concept so designing from scratch isn't possible at this point.

There aren't any humans that can produce 2hp that I'm aware of, but that's beside the point I guess. There are plenty of pump/motors which you can use. Haldex makes nice(GC Series), cheap little guys. I'm not sure what their efficiency is as a pump and as a motor though. Probably both close to 90 percent, or just a little lower.