honestly as far as subwoofers i would go with a lower power number.. unless you plan on upgrading the battery bank quite a bit..
as a avid boater i know that having two battery banks. one for starting and one for stereo/acc is VERY important.
i would stick with two 12 inch woofers drawing around 700 watts total. aside from the mids /highs and i would go with at least a minimum of five xs3100 batterys.. for longevity because hes not gonna idle his boat while he is playin his music at the party spot i would also tell him to consider upgrading the alternators if they dont already have a high output rate. (which they should) because draining those batterys at the party spot and recharging them in a timely fashion isnt gonna be a easy task
thats the downside to having a loud system on a boat. you have to upgrade the electrical ... it takes a toll on your battery and theres nothing worse than being stuck on the water
EDIT:
I AGREE.. GO CERWIN VEGA i can vouch for their sound quality even at higher volumes and their durabilty plus the dude is probably older and can relate back to the vega days. lol!!! bonus!!!