slingo,
Here is what I can tell you. A 10" woofer in a sealed enclosure will begin about .85 cu.ft. external displacement. A 12" woofer in a ported enclosure can easily exceed a 3.0 cu.ft. external enclosure. I use 'external' displacements because that is the number you are most concerned with when fitting the boat. Everything flows from that number. And, there are a number of great woofers in different enclosures in between that .85 and 3.0 cu.ft. The enclosure can actually have more to do with the deep bass extension over the particular woofer. So in a logical sequence the boat first determines the maximum enclosure displacement which in turn dictates the right woofer and enclosure loading method (sealed, ported, etc.).
There are a number of obstacles in planning a subwoofer enclosure.
Your tolerance for leg/foot room loss.
Insertion dimensions and insertion angles past the seat riser and into the underdash cavity.
Any cables that penetrate the sole and are fixed, like a steering cable that should not have its radius changed and should not be placed into a bind.
Room for the woofer excursion.
The elevation between under the dash and the risers that lift the enclosure off the sole to allow for drainage and evaporation.
A heater, heater hose or heater vent.
Venting hose from the bilge.
I know the anticipation can kill you. But the best options will come from taking a few measurements with the help of a pen and pad, tape measure, small level and a couple of squares.

David