You can add ten more batteries but how are you going to recharge them once they are depleted?
So let's say the boat ignition/operations pull 20 amps. The stereo pulls 35 amps (with the transient nature of music). Two big capacity batteries depleted to 11.5 volts, well below half capacity which is bad for the batteries...even a consumer grade deep cycle, and initally demand 20 amps each. That's all of your alternator at high rpms and well beyond capacity at lower rpms. Hmmm.