Lots of great stuff in Largo. You will have to take a tour boat out. The Dry Rocks and Molasses Reef come to mind off of Rodriguez.
Another great area is Sombrero Light off of Marathon. It is surprisingly good and we visit it often.
On our bucket list here before long is to go to Looe Key which is supposed to have some of the best snorkeling and diving in the keys. Bahia Honda is pretty good too.
If you don't mind parting with some coin, take the family over to the Tortugas by Ferry. You can even tent camp there if you want. Gorgeous over there and great snorkeling and diving... but the best stuff is away from the fort and you won't be able to get to it. Still, the old Fort is incredible for the kids. It is one of our favorite places.
Looks us up when you get here. I suspect we will be hanging around Marathon somewhere!!
Dry Tortugas Fort
Kids Snorkeling off of Sombrero (that coral head goes down over 20 feet).