Kinetic to electric back to kinetic energy conversion is not efficient plus the wieight of the generator and motor. It works fine in cruise ships, diesel locomotives because in those applications you get rid of a huge clutch and the complexity of that. Also for the cruise ships they are 360 trainalbe without complex gears, u-joints, etc. For a power boat no real gain. For cars works well beacuse of the gain in efficiency in regeneration in braking. Might be a great idea for a slow cruiser with a small electric/solar combination with diesle/battery back-up. Could also be feasible when there is already a very large electrical demand or a very really powerful way to make electricity like a nuclear reactor (either direct or steam turbine electricity. For the current sub designs we still use steam turbines for propulsion. May change in the future not due to efficiency but radiated noise.