Batteries and amplifier hook up help!

I just purchased a 2008 Bayler 195 F 19. I know my basics when it comes to hooking up sound systems and actually know quite a bit, but now with this boat, i want to be able to power the system while the boat is off and not drain my battery. I want to have the original four speakers that are in the boat still hooked up. I ordered the tower speakers, the Kicker speaker bar which has 4 kicker speakers on it. And also want to place two 12" subs on the boat as well. I will be running it all through a 2000 watt amp. What do i have to do, to be able to run this system, without being stranded at the sandbar with a dead battery!? I do know i might have to add a battery or two, so any help on wiring them together would be great!