The only move Poliwhirl has that is worth Poliwrath having is Belly Drum. You get Belly Drum at level 37. But if you don't want to use Belly Drum (I would suggest not getting Belly Drum) then just evolve it at whatever level it is now.
Above is the competitive answer. For ingame use, evolve Poliwhirl right away. Belly Drum sucks ingame, so don't even bother waiting for it if all you are using Poliwrath for is to go through the game. None of the moves Poliwhirl learns are better for ingame than TMs or HMs are.