The crossover process used in setting a car does not work in a boat. There are multiple factors that will dictate the best crossover frequency in a boat.
1.) Too often we see the crossover used as equalization to correct for a bad sub box design. So it begins with the right box correctly loaded into the boat.
2.) Factor in how low your midbass drivers will go. This will be determined by the model of coaxial (6 1/2" or 7.7", etc.) and their mounting. For example, is the speaker well isolated or on the edge of an open coaming pocket. The high-pass crossover point will definitely influence the low-pass point.
3.) How much subwoofer leverage do you have. Are you running the norm with a single 10 or 12, or do you have something excessive that tends to obscure the normal limitations. In most open field applications the bass output falls off rapidly below 50 Hz, the opposite of an automotive cabin. Its hard to get anything resembling tonal construction with a woofer that is peaking within and limited to a half octave.
4.) Is the objective more towards sound quality or maximum bass emphasis. As you increase the sub gain you're effectively raising the minus 3dB point in respect to the midbass drivers. The actual numeric setting on your amplifier is generally not the true indication of where you're intersecting with the mids. Crossover frequency and gain are very interdependent.
Provided you're not having to compensate for a midbass driver problem, start out with the highest conceivable and symmetrical crossover point of 125 Hz. Raise and lower the subwoofer gain until the woofer best blends in with the coaxials. The bass should not sound remote or detached. From here you can lower the sub frequency, without altering the midbass coaxials, and simultaneously increase the gain slightly with each frequency adjustment. Eventually you'll hear the introduction of a tonal abberation that you don't like, especially at higher volumes. At that point, raise the crossover back up a tad and lower the gain a tad. This will represent the lowest low-pass crossover point and the highest gain while maintaining good sound quality with a fairly seamless transition between the sub and coaxials. Also, use a variety of two or three different music sources.
(Message edited by david_e_m on March 16, 2009)