The power requirements don't change, the hydraulic system is just a way to move the power to where you want it without having to put the engine there. Hydraulic drives are almost vibration free, They can be mounted very low thereby creating a more efficient angle of attack and shaft alignment is simple. Downside is they tend to leak, and the
pumps wear out. I looked at a boat that had one once, engine was in the usual place but drive motor was above the keel.