If you add a second battery, use one of them strictly for starting the
engine. Don't follow your plan to use one for the radio, and then put
the starter, the lights, and everything else on the other.

The most important electrical funciton on your boat is starting the
engine. If you can get the engine started you can recharge a dead
battery hooked to the stereo, etc.

Now, however, if you're a young single guy and you use the boat for
dating- ignore my advice.
Rig that son of a gun so the engine battery is
stone dead about the time you reach some secluded little cove. "Golly,
Esmeralda- it looks like the engine won't start and we'll have to drift
around here for a while until somebody comes by to tow us in..... In
the meanwhile, we can play the stereo and there's some cold beer in the
refrigerator" :-)