Rotom-Wash isn't really used as an offensive pokemon at all, it's used as a defensive pivot. The most offensive set is probably Trick+Scarf.
If you want a pure offensive Water-type I'd look into things like Manaphy, Azumarill, Starmie or Keldeo. If you want an Electric-type attacker look into Mega Manectric, Raikou, or Thundurus. Rotom-W just isn't a good offensive pokemon for the reasons that
littlelucario mentioned.
You can fit it on offensive teams but it's usually defensive or Scarf. There are much better options for offensive Water or Electric-type pokemon.