Google "marine audio", or "marine subwoofer". Many manufacturers offer marine versions of their products, it will come down to what you like and how much you want to spend. I use Kicker subs and PolyPlaner mids/high speakers in my MasterCraft.
I'd try to go with a sealed box, that moist air will do some nasty things to your voice coils/subs. We use polk subs on our boats at work.I believe most of their speaker line is marine rated.They seem to work out pretty good.
cerwin vega has a nice marine woofer. there was a video on youtube of one playing music under water and getting thrown in a pool while playing and stuff