Snorkeling here was awesome! Our whole family loved it. Most of the water is approx 3 - 5 feet deep (close to the island). To see large fish - and coral growing from pilings snorkel around the north coal docks (towards Bush Key). The fish here get huge - and you can almost reach out and touch them.
If your preference is for large coral and you don't mind snorkeling somewhat away from shore, start at the fourth swim buoy (counting from the right) and work your way back to the third buoy. The coral there is fabulous, and while you are not in super-shallow water it's still only 15 feet deep or so - and very clear. Near here there are also two anchors and a chain that links them lying on the ground.
For beginners who want a nice taste of the basics - or for starters in general - snorkel around the moat of the fort. You will see coral and many brightly colored fish and it's all shallow water. The south coal docks have schools of fish, but if you can only do one, do the north docks.
As a warning, some large barracudas swim these waters - they are friendly. We saw one who looked to be 6 feet long (named "Fred" according to the rangers who live there). We also saw many in the 3- to 4-foot range. None were aggressive.
Our best recommendation for snorkeling is to camp on the island for a few days. We stayed 3 nights and wish we had stayed 1 week. When the public ferries are in port, the place has a couple hundred people poking around, so snorkeling can get relatively crowded. Compared to other more accessible places, it's still not crowded, of course, but if you camp there, snorkeling after the boats leave is awesome. Also, towards sunset is when we were able to see and swim with rays (three types) and even a green sea turtle.
We used our own equipment. We have dry snorkels, which are worth the price, especially for kids, but also for anyone who doesn't like that occasional shot of seawater in the mouth when you opt to go under to look at something (or a wave comes).
The Dry Tortugas is a great place to learn to snorkel. Much of the water is shallow and snorkeling is literally right off the beach. We let our three boys go alone (with a buddy) and had no fear at all that they would get lost, attacked, or drown. We saw everything: rays, a turtle, a squid, huge varieties of all sorts of fish, conchs, and many varieties of coral. Actually, the only things we didn't see that we wanted to were sharks and eels.
We love it there and would easily return again.