Flush the power steering system. Power steering fluid is a hydraulic fluid, water doesn't act on the same properties as hydraulic fluids because it can't be compressed. Plus, power steering fluid also has lubricating properties to properly lubricate small worm gears in the steering rack/box and pump. Water will cause rust. So flush it out completely and install the proper power steering fluid if you ended up putting a ton of water in the system.
Flush it as soon as you can, it could burn out the pump. Depending how much water you put in will depend how soon the pump fails.
You can wreck you power steering pump, Power steering pressure lines and basically the entire power steering system. If it is a front wheel drive this would extend to the destruction of the Rack and Pinion system.
You'd lose hydraulic pressure, and risk rusting out your steering system as The Count says.
I put about 1/4 cup in my power steering fluid...will I be OK