"Larger recreational vessels, owned by U.S. citizens, may (at the option of the owner) be documented by the U.S. Coast Guard. (Click here to read more about obtaining the U.S. Coast Guard’s “Certificate of Documentation”.) Documented vessels must also obtain a Florida registration and display the validation decal when operating on Florida waters for more than 90 days."
So basically, as long as you run out to 3 miles from the coast line once every 90 days (or take the boat to another state) you can have it registered under USCG in FL.
As someone mentioned, it's all about the money and revenue.