Is it for the cockpit or cabin?
If it's for the cockpit I'd just get a 60 quid special from Halfords with a standard plug fitting because in all the years I have had boats stereos last 2 years before the salt and humidity destroys them when in the cockpit. You'll be surprised how bumpy a boat is when trying to adjust radio while on the move. Get one with big control buttons and USB input. Taking CD's to sea is a bad idea but burning them to USB stick works well
I put this one in. 50 quid. Plenty loud enough to be heard above two big diesel engines roaring at 3.4k rpm and 23 knts. Small buttons are a right PITA when scrolling, so is volume knob hence my advice. Sound quality actually rather good for the price. (I do have good speakers aboard)
http://www.halfords.com/technology/car- ... car-stereo. I have a lovely Bose system inside the cabin, but it never gets used. C'est la vie