Here are photos of a subwoofer enclosure inside the port side helm locker and the related vent. This came from Earmark's installation gallery. The enclosure is clad in marine-grade, non-mildew carpet with HDPE stand-offs underneath to allow for drainage and evaporation, and it uses aluminum mounting brackets. The 10" woofer faces aft into the vent. The enclosure is about 0.75 cu.ft. internal gross which is slightly larger than what is needed for most of today's air suspension woofers (something with a Qts around 0.5 to 0.55.). The enclosure was built to dimensions allowing insertion through the pass-thru hatch.
http://www.earmarkcaraudio.com/install_yamahaar230.asp
There are considerations with this approach comparing it to a direct radiating infinite baffle for example.
Pros: An enclosed air suspension woofer uses the trapped air in the small sealed enclosure to control the woofer cone for the most part. This is a more linear form of suspension than infinite baffle so you are going to get more excursion capacity under control which translates to more maximum volume....if you have the added power. You can run a larger woofer, often going from a 10" to a 12". With enough extra woofer surface area and power you have more acoustic leverage in an open field boat where bass energy tends to dissipate like flash evaporation. However, this approach also has it's challenges.
Cons: You are essentially placing a sub/enclosure inside an expansive bottle. This should be an easy visualization. The locker and adjoining gunnel cavities make up the bottle. The bottle is huge which means it's a compliant air mass. The bottle's mouth (vent) is relatively small in surface area and that represents resistance. First, you want a vent opening that at minimum is equal to the surface area of the woofer. Second, you want the vent to be as large as possible as compared to the volume of the bottle. An expansive bottle (locker & adjoining cavities) and small mouth (vent) is a bad ratio. Essentially this becomes a filter. The helm locker chamber is far too large and leaky to behave like a resosnant chamber giving a boost like an acoustic guitar body. The result of a bad ratio is a) the bottle absorbs and releases bass energy with a long phase delay once it flows from the vent, b) the direct radiation from the woofer's face and the time-delayed radiation from the collective bottle do not reach the vent in phase, c) there isn't much impetus to drive the bass energy through the vent, d) the subwoofer and sub amplifier will work much harder to produce the same output in the cockpit, e) although there will be SPL losses, you will still get a great deal of bass energy coming through which will be heavy on tactile feel, rumble, thump and vibration, but a major filtering of midbass, attack transients and pitch accuracy will occur. This tends to be more of a boat shaker than a musical bass maker.
So much of the result will depend on which scenario is better on a particular boat and how the design & installation is executed. Both approaches have their place.
I can't argue with the logic behind two woofers and double the surface area. Surface area is by far the most efficient means of gaining output as compared to more excursion/power. The sub amplifier and subwoofer that operate the most conservatively always sound the best.
I might also mention that tuning plays a major role in bass performance. What works best in an automobile does not translate well to an open field boat environment.
