Raypanic, you didn't mention adding an electronic crossover to your system. Adding an electronic crossover helps considerably. If you only have a bandpass filter in the sub and feed the rest of the system full range you have some real room to improve your system even more with minimal investmet.

Technically speaking, an electronic crossover gives 3 db effeciency gain over passive ( coil and capacitor ) crossover and even more over a simple single bandpass filter system.

To gain that 3 db you have to double your power. To gain another 3 db, double again. That puts the efficiency of the electronic crossover into perspective. It effectively represents doubling your power or maybe more in your case.

Also, if you have any way to physically couple your sub to the boat, it should improve low frequency output.

Bolt (not screw !) your sub in with heavy rubber washers between the sub cabinet and the boat.
Good luck and good listening.

guys check out WWW.UBID.COM they have new and refurb gear for pennies on the dollar! I have seen new bazooka subs selling for well under $100.