I ran a car audio shop for many years the one main thing I would recommend is a very good head unit my preference has always been alpine they do not make a marine unit but the tend to hold up better and last longer. It all starts with the radio, next you can never have to much power the more power you put to the speakers the better they will sound. There are a lot of nice speakers available I am not real familiar with all the specialty marine brands but most will sound pretty good with a nice amp usually volume is the main issue in a boat as long as it sounds pretty clean. I have always liked the Boston Acoustic speakers, a friend of mine has the bazooka 8 inch full range speakers the sound pretty good, more bass than a 6 inch but nothing sounds like a sub. Just my 2 cents I hope it helps.