First, get Poliwhirl to learn Hydro Pump and then evolve it with a Water Stone. You can get a Water Stone by winning the Bug Catching Contest's first prize, a gift from Fisher Tully on Route 42, or the Pokeathlon dome. All of these methods are before Kanto.
Then, go to the move relearner in Blackthorn City. You need a Heart Scale. Some ways to get them are by going to the Pokeathlon Shop on Wednesdays, Rock Smashing rocks, or using the Pokewalker.
The Move Relearner will then let Poliwrath "relearn" Dynamic Punch.
Poliwhirl learns Hydro Pump at Level 48 and Poliwrath learns it at Level 43, so it should be fine.