this is one of my projects this year as well. WHen I bought the boat there was an AMFM tape player that didnt work, I stripped the speakers out and pulled the knobs off the player, cut the wires...
I have a kenwood head unit that i've been kicking around for a year or so, it's one of our 'rejects' at work, or ones that we've replaced with newer ones...
i definately like the JBL speaker idea though... going to pick me up a pair.