Toastchee, you might want to do a little work online looking at marine or RV batteries and charging systems, or pick up a classic little book called "The 12v Doctor" or "The 12v Bible" from many boating stores.
Any battery that is small enough for you to conveniently lift and carry from your boat to your car, won't have a lot of power. "Lot" being a relative thing, maybe it wil be enough for you. And a battery in a car is literally a bomb, if it isn't bolted down and you are in an accident, it becomes a 40-pound missile filled with caustic acid. You car's charging system is also not designed to recharge it, unless you start modifying you'll just ruin it, and your car battery.
So...if you start over, by figuring out how much power you need and for how long, then you can start designing a whole system. Or, just buy a deep cycle/trolling battery at WalMart along with a charger for it, and see if that works for you.