My mounting options are really very open (building the boxes will be the easy part) and since my original idea got shot, laid dead and has been burned til gone!
Your subwoofer(s) will be the largest part of the current draw and will be one of the initial considerations in sizing your amplifier and charging support, battery capacity, etc.
In sequence, the boat will determine the best location and orientation plus displacement for the subwoofer enclosure. And that will then determine the optimum size of subwoofer and optimum enclosure type whether bandpass, bass-reflex or air suspension. The actual subwoofer driver selection should come later. When you get a chance survey the boat and scout the possibilities.