Water poured in power steering reservoir

Depends on how much is a little. If you needed to add, it suggests there is a leak and something must be replaced. That is the time to drain all the power steering fluid, then refill the system when the repairs are done. Otherwise power steering fluid is not changed for any regular maintenance.

If a small amount of water was added, say half a cup, for example, it is probably not enough to worry about. It will not compress under pressure, which is what we need power steering fluid to do, but it does not provide the same lubricating properties. That is not a big concern in the rack and pinion steering gear, but it is to the bearings in the pump.

Over time the water is going to vaporize and work its way out of the system, but you will have bigger problems if the temperature goes below freezing. Ice in the pump could break the vanes that do the pumping, when you start a cold engine. If ice forms in the spool valve in the steering gear, which is what controls the direction of pressurized fluid flow, you could be unable to turn the steering wheel. Heat from the engine will eventually melt ice in the spool valve, but if that ice breaks the vanes in the pump, the damage will be done, and only a new pump will solve that.