NEVER, EVER run your boat while at rest to get a charge when listening to the stereo.
Alcohol consumption amplifies the poisonous impact of carbon monoxide. Combined, it takes much less of both to kill someone. Add in stagnate air and you have an even greater risk. Several years ago two people died in Oklahoma from this exact practice. They passed out, fell off the swim platform when no one noticed and silently drowned.

You can have as many batteries as you want. Fully charged batteries represent no extra load to the alternator. However, and this is a big 'however', discharged batteries are a liability rather than an asset. A number of highly discharged batteries or an inordinately large battery capacity that is highly discharged combined with the stereo draw will have an alternator running as hot as an exhaust manifold. And btw, as Mike mentioned, an alternator at idle doesn't produce very much current. What can be worse is when exhausted batteries are used beyond their lifespan. This is extremely stressful for an alternator and will lead to an early alternator demise. If everyone only discharged their batteries no lower than 12.0 volts and batteries were replaced when needed and on time, alternator rebuilders would be put out of business.
If you have a large system that you play at rest for a protracted period then it is not practical to burn a tank of gas to fully recharge. You must have SMART AC shore charging to fully restore, not to mention fully condition.

David