If the goal is to be BIG in the cove for more than 2-3 hrs, 4 31 series batteries in your stereo bank minimum, PLUS a start battery. Those bigger 10s and 12s will consume substantially more power than someone bragging how long they can rock with some 6.5's (of course you will dominate their sound) Big surface area requires big power to move it.
Example - My RF 1500bd1 first powered 4 10 kicker cvr's, then I stepped up to 2 15" L7s. The 15s move WAY more air, louder of course, but my batteries draw down much quicker. BTW, I run 2 optima blue 34 and 3 kinetic 2400s backed up by a genny and power supply. I have the experience and been though the learning curve, just trying to give you a heads up.
4 31 series deep cycle or 2 4D deep cycle (check out Interstate). The 4D will last longer due to the general characteristics of a single larger battery out performing several smaller of the same total rating. Don't believe me, check out Earmarks x80 install where they used 4 6v golf cart batteries (also and option). Btw, Kinetiks are sweet but Expensive. I got all 3 for $400; installed less than a month, from a guy going away to jail; opportunity
Make sure you have a 30+ amp charger and plug in every night.
It never fails, you get 3+ hrs in, party is rock'n and voltage is dropping. Buzzkill. Then douche mode sets in and you start the motor to charge the batteries. Motor creates noise so the music is not as loud, carbon monoxide danger, at idle alternator does not put out more than 30-40amps so you will end up burning up the alternator. Idling for 1-2 more hours burn a CRAP LOAD of gas that at $4/gal you could have put into more batteries. I have burned $50 easy in gas keeping the afternoon party going before.
$6k is already spent on equipment why cheap out on another couple hundred that will let you continue to enjoy the system and boat? (not to mention, the less often batteries are depleted, the longer they will last)