murph, I'm not thinking about the boat or boat registration, but rather that someone is going to say "Yes, well, you left the country, now you need to reapply to reenter, your visa didn't cover multiple entries."
Ah, yeah....they're especially uptight about that in Florida. I'd expect that as long as you stay inside the 12? mile limit? they might consider that you haven't left the country, but hop offshore to the Bahamas, or out into the Gulf...I have no idea what the visa regs are. The only aliens I know are in Area51 and they only answer to the MIB. (G)