We have basic West Marine waterproof 5 1/2" (I think) mounted just where you describe in hull #62. The job was nicely done (I didn't do it!). They are placed just foreward of the locker doors so that they do not show in the interior.
The sound is actually not bad, although it's real hard to hear on the rail in a blow without deafening the helmsman. We have a kill swith in the back of the cockpit in easy reach of the helm. Comes in usefull when you have a set lined up for just after the start. (I love my CD burner and MP3 collection. We've been making new disks for each race.)
Bose makes some nice (very small) fully contained speakers for outdoor use that we've been hallucinating about mounting on the underside of the spreaders. You could ensure that your competition gets to enjoy your music at important moments. Anyone ever seen a setup like this? I think it would be much easier to hear on the rail, and much easier on the helmsman's (my) ears! However, I fear that vibration in the rigging would make thse very short lived.
