24' really kind of is the cutoff size for a dedicated stereo. I'm with you though I'd like tunes while sailing.. I get enough charging between the outboard and a small solar panel that as long as the stereo isn't a 200watt monster, its pretty easy on the battery.
I was going to get 4 standard inspection ports larger than the speakers I am getting. I need to put inspection ports in anyway, to get to my motor mount and the new swim ladder I am installing. So I figured I'd buy 4 inspection ports... install 2, keep 2 for the extra plates. I'll install the ports where I need them on the transom and do the work I need to do... when I am done I'll cut the speaker holes into the plates and mount the speakers... at any point if I sell the boat I'll provide the extra plates without speakers as part of the deal. This is on a long list of projects I am doing to my boat this winter.
I also agree with others that if the head unit is below, a standard car unit is fine. Definitely want a remote, but I'll add that the remote needs to be able to easily hit the unit from the cockpit (facing you). I'd do wall mounted speakers for down below, like bracketed bookshelf speakers.
I'm also going to opt for NO CD option for the stereo... just needs bluetooth, AM/FM and some kind of USB or SD card option as well as AUX in.
I'll link to the stereo and speakers I had on my list..
JVC KD-X310BT Digital media receiver at Crutchfield.com
This are tiny speakers... so that I can use smaller inspection ports (6")...
Kicker KM44CW 4" 2-way marine speakers (4-ohm) at Crutchfield.com
Also these seem to have a moderate amount of shielding around them. going to experiment to make sure it won't interfere with the tiller pilot as they'll be about 3 feet from the tiller pilot.
I used 4 ohm bookshelf pyle speakers on my last 25 footer, mounted them pointing towards the cockpit, using a 40watt stereo... and it was pretty quiet when the winds were over say 12 knots, and it drained the battery quick... I wound up adding a 100watt amp so I could hear it, which of course drained the battery quicker.
So the idea is to get the sound closer, therefore not as loud, therefore not as hard on the battery.
I've used several bluetooth speakers, and honestly, none of them impress me, and a good one will run you more than what I am proposing doing.