ActiveCaptain is a good resource
Me, I'd cross at the Channel Five bridge (65').
There is a bascule bridge at Snake Creek (Islamorada) this is what AC says:
"Phone: 305-664-3637 There are overhead wires across Snake Creek. The Florida Keys Electric
Coop has advised that the lowest wire is 75 feet and does not carry power, but disclaims any liability. A local boat with a 71 feet mast
is reported to cross successfully. A boater crossed through on a Lagoon 400 catamaran
with a 70 foot mast
with antennas on July 4th, 2014 and had no issues with overhead power wires or depth
(they have a 4 foot draft). Bridge tender
responded within seconds to their call and opened on the hour at 10 am."
I seem to recall
there can be quite a current
running at Snake Creek, which is why I never attempted it.