yeah well i plan on using that battery you suggested a while back....i got three of them.....should do the trick....i 12 would be a little big i think and havnt seen any 12" marine subs out there.....I already got the amps batts and head unit.....just need some speakers.....or i just might go full range with some coaxials.........As far as the sub concept goes, I'd do a really small 2 channel amp, like this: US Acoustics USX 2050 amplifier - eBay (item 180591232069 end time Mar-23-11 00:13:03 PDT)
And get a cheapy 12" sub so you can get as much cone area in that thing as possible. A little amp like that + a HU isn't going to put much strain on a decent sized batt.
You have no idea how much fun it is sir........That actually sounds like a good time!! IMO, I would forget the sub and just run a small amp on what ever fullrangers you put in there. Now unless you are blasting it full tilt, a good battery should last a while. I dont know about 9 hours tho...