Umm, keep in mind that you are talking about running 120V current from a generator through an extension cord over the water and into your boat? Safety wise, that sounds like a potential disaster, especially if partying and drinking is involved. Big difference between the 12V on your boat and a generator, especially if you're spanning water. I think a far better choice would be to upgrade to larger capacity batteries and make sure they are fully charged at the end of the day/overnight with a larger alternator on your boat as a good option as well.
Maybe I'm off on this, I defer to those of you with far larger and more complex audio systems than me, but water, alcohol and 120V current just doesn't seem like a good mix to me.