Re: Possibly dumb question about Battery and Stereos.
I/O's have alternators, basically no different than a car electrical system. SOME outboards have alternators, but the majority do not. The charging system on the non-alternator outboards is good for 10-15 amps, barely enough to keep the battery charged normally...
If you plan on listening to the stereo a lot with the engine off, put in a dual battery setup with a battery isolator. The bigger question is what kind of stereo? A simple head unit with 4 speakers can go A LONG time on a battery. I've also built stereo systems that would flatten a battery in 15 minutes tops.