small boat.... I used one of the iPod boom boxes...they were great, and even on battery power (recharge) would last most of the day.
a bit bigger boat, with batteries and charger.... I use regular car stereo decks and either alpine or polk speakers. Theses decks last as long or longer than "marine" decks. Get one with a remote if you can.
big boat or my liveaboard boat.... I just used the decades old Bang&Olufsen 5000 Receiver and wired speakers where I wanted them. I had a switch to tap in to the Hunter audio and my audio. I added an ipod dock and really used the remote.
It is kinda hard to beat the bluetooth speakers coming out these days.. I've always installed speakers behind inspection ports on the boats we've owned, and that works well if you really want those kinds of watts, but it is a lot of work..
This has already been hinted at by the previous replies, but I'll express it clearly; don't put unnecessary holes in the boat. You risk creating leaks that will drive you nuts, and lower the resale value of the boat. Cockpit speakers on a 24' foot boat are unnecessary.
Go with a boom box (~$50), or bluetooth speaker (>$150). NO holes in boat, NO wires, NO installation headaches. When it breaks, throw it away, and buy a new one.
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.
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.
One more piece of advice, keep the speakers and the stereo faaaaaar away from the compass. Don't permanently mount anything till you see what the effect on the compass is. Yes, I learned this one the hard way.
Also, if you're going to mount speakers in the cockpit, pay attention to where people normally sit. I mounted mine on the cabin bulkhead, and it turns out that sitting against that bulkhead is the most popular passenger position, blocking the speakers.