1.a few things you can do..
a. buy an amp that offers a preout. This way you can daisy chain the signal between 2 amps. I'd make them the most popular living areas.
b. Buy a 2 to 4 channel eq that will allow you to divide the signal between 2 amps
c. least expensive is to use a y adaptor to split the rca's from your deck's preouts
2. I have a question first... Where do you plan to put the subs? accordding to your question I'd run 2 subs from a class d mono amp... forget 2 channel class a/b amps for bass... depends on the gear, though...
3. playtime always depends on the user's finger.... play loud, don't play long.. Since you're putting it on a houseboat and plan to charge it at the dock only, you should consider using 2 6v batteries.. You could get a whole weekend of playtime if it's at consertative levels...Cranked loud (my definition) continuous probably 3hrs depending on how many amps are on... Basically, your question is raelative under several circumstances...
Also, you could put line level attenuators on all of your amps inputs... This would enable each area to have volume control.. actually you could put inspeakerline home style volume controls as well...
Please bug... It's what this forum is for.... Plenty of current and ex audio installers here to help out