agree with David. half the fun of building a custom enclosure is doing the calculations and knowing it's coming out close.
personally, never sweated a 1.2cuft box for a sub optimized for 1.3cuft. a boat is such an acoustically bad environment and quite noisy at that. I would bet that most would not be able to discern a speaker put in a slightly "off" box based on the environment, but even moreso on that fact that most don't pay to have systems professionally tuned to drive the speakers to their optimum efficiency.

my .02