I figured you were going to run a generator to keep the batteries charged since you aren't running the stereo batts on the alternator....
I'm with HH, you'll want the 2ga...
If you don't run it from the perko like HH suggested, you can put a circuit breaker inline that you can trip when the boat is in storage...
How long and hard do you play your stereo?
I don't play sandbar dj...maybe 50-60 % most of the time.
If I run the inline circuit breaker, where does it go?