Abstract: A hydraulic system which includes a pump for supplying hydraulic fluid to a load by a fluid supply line and receiving hydraulic fluid from the load by means of a fluid return line. The pump, as well as a drive motor therefore is enclosed within a pressure vessel, having a gas interior which forms an acoustic impedance mismatch so that the pump is acoustically isolated from the surrounding water medium. The fluid return line is connected to a reservoir having a moveable piston which separates the reservoir into two compartments, one filled with oil and the other with gas. The reservoir is sealed so that its internal pressure which is relatively low, for example between 10 and 100 psia is completely independent of the pressure of the surrounding water medium and maintains the return line pressure at that same low value.