CUSTOM STERIO! what sounds good on a boat?

I just bought a 20ft boat with stock everything. I'm a car sterio guru but I have never owned a boat. I am wanting to put in 2 or 4 new 5"-6.5" speakers with my old Rockford Fosgate amp and a head unit. I have two speakers right now that are by the driver seat and passenger seat and I am going to replace these ones with two new ones. I am also thinking about installing two in the rear and maybe two in the open bow for a total of 6 speakers throughout the boat. would this be a good idea? Just curious as to what boat owners use and does any of your custom sterios sound great in open air on the water? I'm thinking about purchasing a Pioneer deh-p7500 cd/mp3 player, and thought about maybe either pioneer or sony xplod's speakers. Boat Gurus?????? Thanks.