There certainly is nothing wrong with Brian's system design approach and that works for many people.
After having a dedicated marine division since the late 90s and after installing several thousand boat systems we appraoch it differently at least in how I would describe it.
We normally treat the subwoofer as part of the in-boat zone just like a woofer in a three-way system. Why? First, because in most moderate systems its not practical to drive the subwoofer to the level that it can project to the rider at wake range. In most moderate systems the woofer and amplifier would be over-driven not to mention that the in-boat occupants that would object to that much SPL in their face for any duration. Second, when lounging behind the boat for any period at rest it takes an inordinate amount of power to drive a sub from out of the bathtub, so to speak, away from the boat when the listeners are below the deck cap down at water level. So many people go easy on the sub in this scenario to conserve battery power. Lastly, we are very focused on sound quality in the boat so we always try to maintain a very balanced in-boat sub/satellite system in respect to the product and in the tuning process. The tower speakers are often used so differently that we address this as a different and independent zone. Plus, we would prefer that boaters select larger tower speakers with a warmer balance so that they at least capture enough bandwidth to simulate fullrange. To a degree we do consider the ambient radiation from the tower speakers in the boat.
Now there are definite exceptions. There are those boaters were we design and build their system specifically to entertain others at the sandbar or dock. They usually want the entire musical range including the bass to project along with the tower speakers for several boat lengths away in a dominant fashion. Under those circumstances with a very powerful system we do have to consider the subwoofer in unison with the tower speakers. It takes four times the power to reproduce one octave lower bass at the same amplitude and it takes four times the power to double the distance at the same amplitude so with those types of challenges this isn't an approach for a moderate budget system. And in that case you are looking at a mighty big sub and enclosure, mighty big sub amplifier and a mighty big battery bank.

David
Earmark Marine