I think a harpoon with a rocket attached to it would be hilarious, and also useful in many situations. Imagine shooting an enemy ship in the side and they get a side movement vector for the next 10 seconds or so. I see squids spinning like dervishes from off center, side hits. =) Shoot a ship from a higher angle and force it downward. Shoot it on the front and slow it down, etc. Silly but fun, and very much in keeping with the more fantastic aspects of steampunk fiction.