Omastar 93% with water gun and hydropump worth power up?

I would say keep it for prestiging gyms against Fire Pokemon unless you prefer style points over using a Vaporeon.

Water Gun / Hydro Pump is pretty much your standard water moveset, but what Omastar has over other water types with the same moveset is double resistance to Fire, which means a 1000 CP Omastar would have an slightly easier time fighting a 2000 CP Fire type than another water type Pokemon with around 1000 CP.

If you power it up for attacking, it's basically just a weaker Vaporeon.

Consider leaving it low for prestigeing gyms, there are better attackers but due to it's double resistance against fire Omastar is ideal for training against much stronger fire Pokemons