We dive but have young boys plus grandparents who love snorkeling. We've gone on three $35 tours here and they have all been great. We've also always rented houses on the ocean side of the keys and snorkeled a LOT off the piers. I bet the bay side is also good. If you have access to a pier or a walk-in beach, you can easily snorkel for free and find plenty of neat stuff. In January we saw lobsters, rays, seahorses, angelfish, barracuda, and a nurse shark within 100 feet of our house in Islamorada.
FYI, almost all the dive ops will take snorkelers out (it's one way we get in some diving- the grandparents snorkel with our boys). Many of the reefs in the upper keys are shallow enough for fantastic snorkeling.
If you're going to be snorkeling a ton I would make two recommendations:
1. beach shoes that fit into the properly sized fins for walking out through the sometimes yucky seaweed stuff & rocks & garbage that washes up, and
2. a shorty wetsuit. Maybe the wetsuit is overkill, but with our 8 year old swim team member we have a much easier time and he stays out longer with an added touch of buoyancy. In January I wore my own wetsuit and he could just crawl on my back when he got tired. We got him a used old faded perfectly useable kids shorty at a dive op for like $15. They often have old rental stuff they'll sell for a song, and the kids stuff is very lightly used.
Have a great time!!!! It's an incredible place.