Systems and method for operating subsea devices and pressure compensated reservoir systems useful therewith which in certain aspects, include a chamber with a piston therein acted on an exposed side by water, e.g. sea water, to provide operational hydraulic fluid for operating a subsea device, with a piston rod having an end in a separate chamber acted on by a fluid to compensate for a pressure differential between the pressure of the water on one piston side and the pressure of the operational hydraulic fluid on the other piston side.