Do a little homework on acoustics before you worry about super audio systems and lots of watts. Underway, there will be so much ambient noise that you will never hear the fidelity that high end sound systems produce. Trying to compete with a ridiculously loud engine is a waste of time too.
Buy a basic marine sound system that can produce enough volume to enjoy on your boat at anchor. If you are trying to get enough volume for everyone within miles to hear it too, you have made your first mistake in boating ediquette.
To each his own, but if I am enjoying a nice bit of peace and quiet on the bayou, and someone anchors next to me with some silly sound system that is annoying the heck out of me, I promise that I will have to test my boat in a tight turn around them, at maximum wake speed, over and over and over and over ......
Having been in the music business for quite awhile, I might also have to find out where they live, show up with a 30 KW genny and enough sound gear to melt their damn boat in the driveway!