Mona Miller stated that, yes, they do sometimes migrate down through Florida and across the gulf of Mexico. They are known to island-hop, land on ships, and make their way through the Caribbean Islands. Dr. Urquhart confirmed one by tag that was from the U.S. that went to Cuba. http://www.monarchwatch.org/tagmig/urq1.htm
She also mentioned that the four maps (click on the thumbnail images to view maps) show the significant recaptures (304) in the United States that were reported in Urquhart's "Insect Migration Studies" newsletter from 1964-1994. Most interesting among these recaptures are the instances of Monarchs in Florida crossing the Gulf of Mexico to Texas and Louisiana and one Monarch recovery in Havana, Cuba!
Male Monarch on Exuma Island, Bahamas
Mona Miller recommended the book "Monarch Butterfly, International Traveler" by Dr. Urquhart has some more details. She also wondered if I took any photos, since there are Monarch Latin American cousins that look a lot like them. You can also find information about these cousins of the U.S. Monarch in Dr. Urquhart's book. Please contact me about the photo; I'd like to know if it could be a Latin American cousin to the U.S. Monarchs.