We signed on to the Discover Scuba course. They advertise 2 dives at a reef after the introductory lesson at a pool with your day ending at about 4:30pm. Ours ended at 2:30pm
The day of our instruction was apparently too choppy to go to the reef so they substituted a place called the "lagoon" instead. I was told that it was calm, had a small reef with some lobsters and fish, would see an an archelogical site, and see the Jules Verne underwater hotel.
This all sounded like it would be a satisfactory substitute but I should have asked more questions. The lagoon turned out to be a cheap manmade enviroment off a boat canal that was virtually void of any fish. The water quality was terrible (the instructor said after the dive that some of the boat residents dump their toilets directly into the water and recommended eardrops to prevent infection). The underwater structures (hotel, cannon, ship ballast) were hardly visible and boring.
I will have to say that we saw more snorkeling along some of the beaches than what we saw during our scuba trip. If you want to see schools of fish, crab, rays, and clear water when your in the keys I would recommend snorkeling off a public beach at high tide. This is much cheaper and much more enjoyable. Another option is the guided snorkeling tours ($35 versus $135 for Key Dives Discover Scuba).
I am also suspicious of the excuse that it was too rough for the reef dive because of the wind, there is always a breeze coming off the ocean side. I think that if they have a boat full of experienced divers going out that they like to keep the beginners off the boat. I'm sure we would have slowed things down and got in the way.
Just a couple of more things. I was charged a fuel surcharge (we didn't take a boat ride) and our instructor stiffed us for his lunch.
I hope this helps some other travelers make a better decision.

After diving in the Keys for nearly 15 years, our operator of choice closed, leaving us with the dilemma of "where do we dive?" After researching the internet and calling to speak with a number of operators, we decided to give Key Dives a try.

Their boat, the Giant Stride, a recently acquired (December 2006, according to the owner) Newton, is well-designed for diving. There is a table for photography equipment (and a separate rinse bucket for camera gear), and even when the boat was full, we never felt over-crowded. We had up to 3 ft seas during our trip, and the boat was quite stable in all conditions.

Key Dives offers guided diving on every dive, which is a plus if you have a group of divers with varying ability levels, or someone who hasn't been in the water recently. The per diver cost is only $10 more than an unguided dive. Of course, the down side of diving in a group is that you could get stuck with someone who is an air hog, forcing the entire group to surface early. We did a guided dive to a deep site that was new to us, thinking the guide could point out the highlights of the site, and we'd come back unguided later in the week. We had an air hog in our group, and we were all forced to swim high over the reef in a stiff current on our return to the boat. To the guide's credit, the diver who was low on air was sent up the line when we got back to the boat, and the remainder of the group was allowed to hang out on the reef. Dive times averaged 60 minutes (or per your computer, on deeper dives).

Morning dive trips are to deeper sites, with the afternoon trips being shallow destinations. They don't go to the Spiegel Grove (too far from their location), but will suggest an operator in the Key Largo area, if asked. Multi-day packages are available.

We found the staff to be friendly and helpful. They'll take your gear to the boat and set it up on your tank as part of their routine service, whether you're diving guided or unguided. Since we're pretty self-sufficient divers (just tell us when the pool is open and when to be back on the boat), we like setting up our own gear, and the staff is happy to honor your preference. The staff also did a good job of keeping snorkelers out of the way of divers, even when the boat was at capacity. Another plus - they rinse everyone's gear at the end of each trip, and will label and store your gear at their shop for the duration of your trip.

One day, the owner heard the visibility was better farther north than the planned trip, and he announced that we'd be making a longer run - and the excellent visibility on those dives was more than worth the longer boat ride.

Nitrox is available. There is fresh drinking water on the boat, with a cooler available for your own drinks and snacks, and fresh fruit was offered after each dive.