Loee key is, personally, my favorite reef in the keys. I've spent alot of time diving around Big Pine key, and the area is really nice. Largo is good, but I tend to find it's a little more touristly (relativly speaking, these are the keys) and expensive. Snorkeling from the beach is going to be a tough order to fill, since the keys are a barrier reef system. I know at largo you don't start getting nice patch reefs until you're at least 2 miles off shore. For reef diving a boat will be necesary. That being said,there are other ecosystems in the keys. I know of some nice mangrove area and a spounge flat that is fun to dive.
Hope this helps