Just got back yesterday from a week stay at the Courtyard Marriott diving with Rainbow Reef Dive Center, which was well-regarded both here and on Trip Adviser. Here's a breakdown of how it works.
1.) Each boat trip does 2 dives. I think a trip is around $75 or so; there are package discounts; I did the best package rate, 10 boat trips (yeah, 20 dives), which made it $60/trip plus tax. I paid $644.95 at the end, for the diving (not counting tips).
2.) The boat goes out in the morning around 8 a.m.; you'll have booked in advance, but check in with them. The first dive may be a deep dive (e.g.: Spiegel Grove, Duane, Bibb), followed by a shallower reef dive. Figure on a 30 to 40 minute or so boat trip out to the first dive site, shorter to the second.
3.) A free dive guide is put in the water; 2 if it's a pretty big group. You don't have to follow the guide if you don't want. The Captain will likely tell you something like 'pool's open,' please be back in an hour.
4.) Boat should get back somewhere in the ballpark of noon; you'll likely get a 30-60 minute break till the afternoon boat goes out. If you stay at the Courtyard Marriott, that's time to hit your room and eat something there.
5.) The 2 afternoon dives will be relatively shallow reef dives. I'm talking mostly 20 - 40 feet deep.
6.) Viz. my week was around 30-50 feet. Not was good as Bonaire or Curacao. On the other hand, I saw goliath grouper on the Spiegel Grove and Duane, and both nurse sharks and reef sharks on the reefs, and I'd never seen either in Bonaire. I saw quite a few black grouper, 2 Nassau grouper, a bunch of large barracuda, several big green moray eels, and on some reef sites a whole lot of fish.
7.) Staff were fun, friendly, helpful (e.g.: often changed my gear over from one tank to the other after the first dive), safety conscious and the dive briefings were practical and helpful (from both Captain and the guides).
8.) When we did the wrecks, we giant strode in with a 'granny line' in one hand (a rope from the back of the boat around it to the mooring line out front), went around & went 'hand over hand' down the mooring line to the wreck; came up the same way. Ropes didn't have anything on them that stung me, but had plenty of barnacles; you might want a pair of gloves.
9.) You need AOW to do the deep wreck dives; couple of guys didn't get to do them. They were asked if they had log books showing recent deep dives, and they didn't. Not sure what might've been worked out if they had.
10.) Tanks were 80 cf aluminum. They had a pair of 3300 PSI 100 cf alu. tanks available by special request, and that's all I dove with all week. My fills per my Atomic Cobalt were around 2800 to over 3000 for the week, but my Oceanic VT3 tends to give PSI listings about 70 higher than my Cobalt.
11.) When you end a reef dive, you come up behind the boat & grab a rope, take off your fins, and pull yourself to the ladder. In this shallow top water layer, we encountered moon jellyfish; dinner plate-sized jellyfish with short tentacles that can sting painful. I'd use a fin in hand to shove them away from me. Seldom saw jellyfish down on the reef; mostly an issue getting out (and that's prob ably what stung me on the legs jumping in one dive, though it felt like sunburn and went away quickly).
To sum it up, loved my 20 dives out of Key Largo, loved Rainbow Reef Dive Center, and the free guide service was a real plus that not everybody offers. But call; they may be booked up when you go. Also, if you're a 'dive your tank' type who wants to dive well over an hour on a tank, this might not be your match.
Richard.