You get it in Platinum as an event Pokemon called Wash Rotom. However, in HG/SS, if you have a Rotom on your team, go to the Silph Co Building. You can now go up the elevator into a room. There are things here that will transform your Rotom into any of its different forms. You can come back here as may times as you want. Wash Rotom is the one that learns Hydro Pump.