Florida recognizes valid registration certificates and numbers issued to visiting boaters for a period of 90 days. An owner who intends to use his vessel in Florida longer than 90 days must register it with a county tax collector. However, he may retain the out-of-state registration number if he plans to return to his home state within a reasonable period of time.

http://www.flhsmv.gov/dmv/faqboat.html#5