Since you said "good snorkeling" you've limited yourself. There's some, but
not much.  In fact, the only good snorkeling I know of in south Florida is
off Lauderdale by the Sea. There are some pretty nice reefs close to shore
in that area. There's some passable snorkeling off John Lloyd Park, at the
north end of Hallandale Beach. That's pretty much it.

There are options other than taking a boat trip. There's decent snorkeling
all along Florida's southeastern coastline.  It's just not right next to
shore. Personally, I'd consider buying or renting a kayak. That will give
you access to most of the near shore sites with relatively little expense
and without having to put up with expense and time it takes to get to them
by boat. You can launch a kayak pretty much anywhere, attach a dive flag to
it and use it like a towable flag, which you'll need anyway, and it's good
exercise.  Best of all, a kayak is allowed places in the Keys that you can't
go with a power boat.  If you choose to try this, be sure to check out the
patch reef right off shore from the Islander Motel in Islamorada.

If that's not your cup of tea, you can also rent boats. It's usually a bit
more expensive than a paid snorkel trip, but you can go where you like and
stay as long as you like. If you've got three or more people, the rental
boat is probably cheaper. You can pick up a local dive site chart or ask the
boat rental people. If you choose this option, there are buoyed snorkel/dive
sites off John Lloyd Park and Lauderdale by the Sea, at Fowey Light, Just
outside of Elliot Key (Biscayne National Park), and all over Pennekamp. One
of the best snorkeling/shallow diving sites in the area is Looe Key. It's
quite a ways south, but is easy to find. If you choose this option, be very
sure to look behind you as you head to the reefs. What you see behind you is
what you will need to find to get back in. A lot of inexperienced boaters
forget to look and have trouble returning to the spot they departed from. Of
course, a water resistant GPS will take care of that problem too.

Lee