Most Rotom-W right now are bulky calm types with 252 HP and 252 SPD. In my opinion, you can't really replace Hydro Pump on a Rotom Wash. While it is possible to run the more accurate surf or the burn inducing scald, this will limit a bulky Rotom W's ability to OHKO threats like Talonflame. Scald as no chance to OHKO, while Surf only has a 56% chance. I guess it's possible if you are running a fully offensive Rotom, but it kind of defeats the purpose of going full out attack if you don't use the strongest move. (Ignore, Rotom can't learn Scald or Surf) Hydro Pump is vital for coverage, is STAB, and in my experience, doesn't miss as often a it should. So yeah, I'd suggest keeping Hydropump. If you post a set maybe we could get a better idea of what you're going for.