If you are interested in adding a subwoofer to your boat, I've got a setup that I'm selling that would do you well.
I have a 10" Polk Audio Dual Voice Coil subwoofer and I also happen to have a kicker 200.4 amp. You could take the amp and run it bridged to each of the 4 ohm voice coils on the subwoofer. The sub would end up getting 200 watts. from the little amp and that would sound pretty decent for the money.
I'd sell the amp and the sub together for $100 as a package deal. (plus shipping). They would be a littler on the heavier side, but I get really good shipping rates through work, so it shouldn't be that bad.
Let me know if you're interested. Let me know somewhere else that you could get a sub and an amp for $100. I think it's a great value. Then just buy a premade box with the bedliner on it and you're in business. (or build your own box of course)