We booked a morning (09:00) snorkel tour. We went to Sundiver's own office ("Sundiver Station" at 102840 Overseas Hwy in Key Largo) instead of buying tickets at one of the many excursion offices all over the Keys. Glad we did, as the staff there were very knowledgeable and helpful. Answered all our questions and gave us good pointers on how to save some money and have a more enjoyable event.
The boat was docked about 1 block from the Holiday Inn, African Queen, and many other of the main attractions of Key Largo. Upon check in, there was a brief wait and boarding commenced at 08:50. We were given an excellent safety briefing before casting off at almost exactly 09:00.
It's about a 15 minute ride to the reef. The first five minutes is a slow, quiet ride up the canal during which the captain gives a very nice informative briefing on snorkelling techniques, emphasizing proper protection of the reef. He was very adamant that NOTHING - not even the sandy bottom areas, be touched in any way by any of us, and gave good explanation why.
The boat anchored to a buoy about 20 metres from the reef. It was an average seas day, with gentle swells about t1 - 2 feet high. All passengers MUST wear snorkel vests, but the option to inflate them is up to the individual. Generally, unless you are a weak swimmer, you do not need to inflate it.
After swimming from the rear of the boat for about 3 - 5 minutes you arrive at the reef and begin exploring. The first thing I saw was a large barracuda slowly ambling along near the bottom. Cool! I saw about 10 different types of fish and probably at lat 8 different types of corals. We had about 45 - 60 minutes of snorkelling time, which was plenty. Most people were done and back on the boat after about 30 minutes or so.
Sundiver has been operating in that location for many years. Their equipment is modern and in excellent shape. The service is very good and safety is clearly their TOP priority, with ensuring their guests have the best possible snorkeling experience the second priority. They are honest and warn you up front that mother nature is in control of the seas, not Sundiver, so various things could affect the comfort of the boat ride, the visibility in the water, etc. But they do what they can to make it as good as possible.