The water should float on top of the power steering fluid (assuming that you haven't started the engine, which will pump the fluid around). Get an eye dropper or even a turkey baster and remove the water and about a half inch of fluid and discard. Top up with clean new power steering fluid.
If you have started the engine, own up as the water may damage the hydraulic seals which means they may fail and you will lose your power assistance when steering, also the internal metal componets may corrode. It'll be a lot easier and cheaper to deal with it now.