Your Poliwag will evolve into a Poliwhirl with ~485 CP. So it makes no sense to evolve it. It'll be cheaper to keep the current one and feed 1-2 candy later on.
Anyway, it's usually better to wait. In this case I'd wait for enough candy to get a PolywrathPoliwrath. Don't evolve too soon. Always wait until you have the ressourcesresources to evolve the Pokémon to its final stage. This way you can evolve the best Pokémon you currently have and don't risk evolving a weak one when you still have the chance to get a better one while farming candy. Maybe you even find a Poliwrath to spend your candy on.