powered sub woofer

Guys Im thinking of putting in a sub before season gets realy going for me.
As many of you know I run a closed bow old school Nautique so space is a premium and exposure to salt water is an issue. I take occasional rollers over the nose and everyhthing gets wet.
I am leaning towards a powered all in one unit for the sake of cost and space. I only need a little base help. Dub step will never be played on my boat. Im old, get over it.
The system is as follows:
Fusion 500 head
PP900-4 amp
8" fosgates front
6x9 Infinities back
It actually sounds pretty damn good but with a powered sub i think i can take some load off the cockpit speakers and get the whole thing louder with a a bit of thump added.
I have looked at the bazooka stuff, JL has one thats like 1200 clams I can stuff it way up under the bow to keep it dry.
Any suggestions?
Thank you,
John