i've taken some measurements on mine and i'm going to do 2 - 12's under the back seat and 4 - 6.5's somewhere up high in the side panels
if you do 2 boxes under the back seat (1 on each side of the batteries) you can get 1.5 cu ft per box\woofer. that's perfect for a pair of low wattage thunder 5000 MTX 12's or something similar. they sound really good in a boat and don't hog your reserve battery power. plus, by having a box on each side of the batteries, you can still get access to them. i just replaced mine this year and they were NOT fun to get out w/ my existing setup.
i currently have 4 - 6.5's and a single 10" sub under my back seat all in separate enclosures... you can barely hear it when it's on a plane.
i'm going to move my 6.5's up front (1 below the throttle and 1 below the passenger front cup holder) then 1 on each side panel in the back next to the back seat somewhere above the gas tank