Epicenter in a boat. If its okay I'll try and answer this one.
An epicenter is an awesome processor for restoring compressed fundamentals to older analog recordings. It works very well in an enclosed vehicle that benefits from a 'cabin effect' which constitutes a second order per octave rising characteristic as the bass declines in frequency. At these lower frequencies the woofer is extremely well damped (controlled so to speak) due to the increasing air mass rigidity in the small and confined cabin of a car, truck or SUV. But here comes the "however."
In the open field environment of a boat without such leverage it requires many times the power and places an inordinate mechanical and electrical stress on the woofer trying to achieve the same output level in the bottom octave. For just a little bit of emphasis in the lowest registers it will suck the life out of the dynamics and tonal construction, especially at higher volumes. Using a free-air would only compound the challenge. I might feel differently if you were running a very serious 15-inch with lots and lots of reserve power.
A more efficient way to increase the overall bass output in an open air boat is to slightly expand the woofer's bandwidth by crossing it over a little higher than in a vehicle for instance. After that, having the right woofer to enclosure to amplifier to in-boat loading combination will make a world of difference.
On another subject, several of the woofers mentioned in this thread are definitely not suitable for 'free-air' applications (like the Excelon or Polk MOMO). What exactly are we describing by "free-air"?