Mostly because I thought it would be cool, I built some multi-level boats. In a few sessions my party is going to be on their way to a distant locale, and I'm prepping an encounter with a pirate ship. My rough plan is to have the pirates use a fairly classic pirating tactic where they attempt to force the other boat to accept a boarding party by using cannons.

Anyway, I've never done a combat encounter on terrain quite this dynamic before, and I'd like to do whatever I can to make sure the terrain adds to the encounter. Can anyone offer some advice about how to ensure the terrain is additive? Or, more generally, does anyone have advice for planning a combat encounter on a boat? (For example, it had only recently occurred to me that the pirates might try to distract the party while a group of them used Underwater Breathing to swim under the ship and flank them.)