To my knowledge Boat U.S. Covers you everywhere they serve. As has been mentioned it's a good value...considering a tow could cost you big $$ without it.
The one tow that I had to call for would have cost me $800. with unlimited insurance it cost me zero.....so it's paid for the 5 years of coverage in my mind..
Sea Tow sometimes has better coverage in an area than Boat U.S. and visa versa..
So considering having both is something I would do for a long trip...
I would also get the " unlimited" version...not the super one that tows you back to your home port...but the level just below.
Running aground is not the only reason, you might need a tow...having an engine
fail outside a tricky inlet, or in a narrow channel....taking on water..etc....etc....running aground is something we all do, and can often extract ourselves from.... but other stuff happens...too..
The other question I would ask, is does your regular insurance cover you in Key West
, and if so, do you have to get out by a certain date?