A lot of this is highly interdependent. So let's start with kind of a test scenario.
Multiple subs are going to really eat up current. Collective fullrange power and lowpass power should be within 3 dB of symmetrical for balance.
More power requires more battery reserves which requires more of a battery charger. It all dominoes. You are only as good as the weak link. Weak links lead to reliability issues and premature product turnover, especially batteries.
You want to keep consumer grade deep cycle batteries discharged no lower than their half power, which is 12 volts.
Playing mostly at what is perceived as half volume is -10 dB and only consuming 1/10th of your full power.
Continuous play for 5 or 6 hours.
Music is dynamic in nature and not like DC lighting.
Class D is highly efficient and that is a big plus.
I would want 200 to 250 amp/hours of battery reserves minimum and a 30 amp minimum convertor class charger so that I had the option of running the Gen to get a charge when needed and still be able to operate the stereo at a moderate level while charging in those rare instances.
There is an answer until every aspect of the system is pin-pointed. Then it can be accurately checked.

Subs? Place the horse before the cart and do some measurements on the boat. Lower deck only. Collected in a single location is best. Concealing the subs would be nice. Perhaps the sub enclosure could double as a table. In an idea situation I would want my sub centered between all my highpass/fullrange speakers. This will provide the best possible phase coherency between the two if you don't mind having a little sound quality as an added bonus.

David