Screw both of them and give it Brick Break or Rock Smash.
Submission is worse than Brick Break, having recoil, doing only five damage more than BB, and having 20% less accuracy. Dynamicpunch has only 50% accuracy and 5 PP, making it completely unreliable.
Brick Break should be used for both ingame and competitive. It has perfect accuracy, does a decent 75 damage, has 15 PP (which is solid), and can break through screens (which while an infrequent secondary effect is helpful when the foe actually uses screens.)
Rock Smash is another option, but only for ingame really. It has the same accuracy and PP as Brick Break, but it does 35 less damage. However, it does have a 50% chance of reducing the Defense of the foe, and since ingame foes don't switch, this can easily end up doing a lot of damage. It is also an HM, giving Poliwrath more use, especially factoring in the other HM attacks you will likely be giving it (Waterfall and/or Surf).
If you HAVE to choose between Submission and Dynamicpunch (which you shouldn't have to. Brick Break you might teach to someone else but Rock Smash is an HM and will never be lost) use Submission, as it has the PP needed to make it through ingame.
