RC airplane props and ducted fans are cheap and purpose built to move fluid at exactly the scale we need for small to medium sized ROV's. Its only logical that we should try using them but the question is always raised " its designed for air, will it be effective in water?" fortunately the answer is YES and I just got bored enough to spend the 15minutes to look up the numbers to prove it.
Reynolds number is a dimensionless number that is the ratio of viscous forces to inertial forces in a fluid.
Re= VL/v V= velocity L= "characteristic length" v="kinematic viscosity"
one application is that the same shape will have the same flow around it regardless of fluid if the Reynolds number is the same. So if we take our airplane prop and put it in water, the velocity is proportional to rpm, the characteristic length doesn't change, and the kinematic viscosity goes from 15.68m^2/s *10^-6 in air to 1.004m^2/s *10^-6 in water. (yes air has a larger kinematic viscosity but remember its on the bottom of the equation)
All you have to do is spin the prop 1/15th the rpm in water than in air and it will be at the same Reynolds number and be as efficient as it was in air.