A lot of hotel employees do internships for less than a year, so as long as it is furnished, you shouldn't have a problem. Definitely try Craigslist (as Kevin said) and pass the word to all the hotel managers in South Beach.
The new restiction is 3 months (see today's Miami Herald). You bought into a tough market friend. 2/3 of Miami properties sold over the last year are non-owner occupied meaning that 2/3 of units sold will be rented out. There is a huge shadow inventory in Miami piling up. I really hope you made a killing in this deal. Otherwise you might be underwater within the end of the year. Good luck.
I don't think it would be EASY to rent it twice a year for 6 month stints- unless the price is very, very low. Of course, it is possible. There are many people looking for south beach, short term housing.
Craigs list is great-Furnish it simply with things that can be cleaned easily. Check with the Chamber of Commerce to see if they have a web page too. I have two rentals in Vero and Blowing Rock and have good luck with Vacation Rentals.com-Good Luck-Pam Mayers-EWM Real Estate