Why not call a custom installer in your area to find out what in-wall or in-ceiling speakers they carry and possibly install in your boat? Most in-wall/ceiling speakers made today are of the weather resistant variety and can sound pretty good if you keep the application in mind. Are you going to be critically evaluating music on your boat, or do you just want to create some ambiance with the ability to rock & roll every once in a while? Speakercraft, Sonance and Paradigm all have good models to choose from.
Good luck and enjoy!
