Abstract: Power supply apparatus having a two part housing, the first part containing a motor in a gas environment and the second part containing a pump in a pressurized fluid environment, the second part having an inlet and outlet for passage of pressurized fluid to and from a tool to be driven by means of the pressurized fluid, the apparatus being of particular use under water, wherein pressure-equalizing means is provided for adjusting the pressure in both parts of the housing simultaneously in response to changes in ambient pressure outside the housing.