We've been asked where to go in the Florida Keys for the best snorkeling, specifically near Key Largo and Islamorada.

Here are some suggestions: 

One of the best spots in the continental U.S. for snorkeling can be found in Key Largo at John Pennekamp Coral Reef State Park. This was the country’s very first underwater park to open, holding its only living coral reef. Spread across 70 nautical miles, snorkelers can see all sorts of tropical fish, sea turtles and even shipwrecks. It’s the perfect place for beginners and families with kids, as tours are available with experienced guides who conduct a “how to snorkel” class right on the boat. You’ll get a safety vest, and if you don’t have your own gear, you can rent a mask, snorkel and fins.

https://pennekamppark.com/snorkeling-tours/

Suggested related books: 
https://www.amazon.com/Reef-Fish-Identification-Travel-Caribbean/dp/1878348698?tag=?tag=rungle080d209-20

From their site: 

Snorkeling

What is required for snorkeling? If you are a good swimmer, you can snorkel at John Pennekamp Coral Reef State Park. They offer rental gear: as of 2021, prices are $3 apiece for mask or snorkel (you keep the snorkel) or $4 for fins. For customers on the snorkel boats, the price is reduced to $2 apiece for mask or fins, or $3 snorkel (you keep), $7 for the set. During the winter months (when the water temperature can drop just below 70), we have "shorty" wetsuits available, for $6 apiece. For those customers who are visually impaired, we have prescription masks available for $5, with lenses running from 1.5 to 7 diopter (for lower prescriptions, the natural magnification that comes from looking through water will usually do the vision correction for you!). All you have to bring is your bathing suit, towel, and sunscreen.

For those customers who have never snorkelled before, we recommend trying the gear out off our beaches first, then heading out on the boats to see the reefs. Remember, there is no real coral growth within swimming distance of the Keys, but the swimming areas of the park are home to several types of fish and sponges, and are a good, non-threatening environment to get accustomed to the gear; once you are ready, the reefs are only a short boat ride offshore! We operate three main snorkel boats in the park, each of which runs three 21/2-hour trips a day (weather permitting) at 9:00 am, 12:00 pm, and 3:00 pm; during the slow season, additional times may be available, inquire at the reservation counter: (305)451-6300. The price for this trip is $29.95 for adults, $24.95 under 18 (please remember that all younger customers must be accompanied by an adult!). All prices are for the boat ride only and do not include gear rental. In-water time is 1-11/2 hours, depending on conditions. All of our boats are safety inspected and certified as commercial passenger vessels by the U.S. Coast Guard.

After arriving at the reef, a class on snorkelling basics and techniques will be given by the mate. This class is optional, but we HIGHLY recommend it for anyone who hasn’t been snorkelling in the ocean before (remember, this environment is very different from a swimming pool), or anyone who hasn’t been snorkeling in a long time and needs to brush up a bit. All of our snorkel boat mates are certified PADI Divemasters and Rescue Divers, and on hand to assist anyone who has trouble in the water. All snorkelers are also required to wear a yellow snorkel vest (provided by the boat) anytime they are in the water; these can be used as buoyancy aides for anyone still getting used to the gear, and are equipped with emergency inflate valves for safety.

For more experienced snorkelers, we do still require the snorkel vest as a safety measure, but you are not required to put any air into the vest if you wish to freedive along the side of the reef. Remember, you may bring your own gear on board the boat, with a couple of exceptions. For safety reasons, we do not allow dive knives or weight belts on board. Because our reefs are protected sanctuaries, we do not allow any pole spears, spearguns, or lobster gear on board. Since all of the Sanctuary Preservation Areas are also NO TOUCH, we cannot allow dive gloves in the water. Cameras are WELCOME, and good luck! If you ask the boat crew, they can often give recommendations for the best spot to get that breathtaking reefscape or awe-inspiring macro shot.

The snorkel boats run to several of our better shallow reefs, including White Banks Dry Rocks, Grecian Rocks, Banana Reef, Garret’s Reef / Cannon Patch, and Key Largo Dry Rocks. These reefs are found from 3 to 5 miles offshore. The Captains will generally run the boat to the reef where you will find the best conditions on a given day, but you are always welcome to make special requests. Depths on these reefs range from 20 feet to zero (several of the "Dry Rocks" reefs stick up out of the water at low tide!), and the captains will point out the best reef sections upon arrival. Please remember that we cannot allow anyone to stand up ANYWHERE except on the boat! Although the area surrounding the reef appears to only be sand, it is in fact an important nursery area for a variety of small fish, crustaceans, and even young corals. Because of this, it is illegal to stand up anywhere around or on the reef, and no souvenir collection of any sort is permitted on the trip. We appreciate your help in preserving this incredibly fragile environment!