Probably the best snorkeling spots in Key West Florida that are reachable without chartering a private boat would be the four main sanctuaries south of Key West. The sanctuaries are called Sand Key , Rock Key, Eastern Dry Rocks and Western Sambo.
They are farthest offshore, which in general often means clearer water and their sanctuary status means larger fish are common and more likely to be visible.
Sometimes in windy weather these spots won’t be accessible and there can be awesome snorkeling a stones throw from Key West but conditions are more iffy.
But it changes often, yesterday the visibility was scarcely over 10 feet in front of Key West. It looked like it was clearing up heading west, so the shallows in western dry rocks would probably be ok for snorkeling.
There is cold front blowing through and the gulf stream is close so these conditions probably won’t last long.