New Guy,
When deciding which subwoofer is ideal for your boat always start with the external enclosure displacement first and go by the numbers. You can back out the materials, woofer displacement and port volume easy enough. In an open field boat the woofer surface area will greatly determine the maximum peak output. But past a certain point it is the enclosure that will mostly determine the deeper bass extension. Too much woofer in too small of an enclosure will result in more upper bass output but a premature reduction in deep bass.
There are other considerations also.
Your amplifier sub channel delivers alot more power into 2-ohms versus 4-ohms so look at the combination and final net load of the woofer(s).
Two 10s are more surface area than a single 12 but dual 10s are more moving mass to drive and control. A good match in thermal power handling is important but a good match for command is a factor as well.
The required displacement for two sealed 10s and a single sealed 12 will be similar. I would prefer a single 12 in a bass-reflex enclosure over dual 10s sealed in an open boat. The single driver bass-reflex will be more responsive and produce more output in the region of the bass that matters most. However, the leap to bass-reflex requires a big jump in enclosure displacement given the added port volume.
The relationship between sealed and bass-reflex is not the same for all woofers.
All the above and sometimes alot more must be part of the equation. Get with Phil from Kicker and see if he can put you in touch with an experienced Kicker dealer in your geo area. A good dealer will automatically take all this into account along with your particular boat and your personal preferences.

David
Earmark Marine