Gee, you just missed the Caribbean 1500 Rally, which left Hampton, VA on November 1. You have time to meet the fleet at its finish in Tortola, BVI. Caribbean 1500 rally
The Virgin Islands, US and British, are a good place to look for cruising boats. Next would be Rodney Bay, St. Lucia, when the Atlantic Rally for Cruisers fleet arrives there. The boats leave Las Palmas de Gran Canaria on November 21, but I wouldn't recommend you fly to the Canaries to try to pick up a boat. But check info out here: ARC 2010
Philipsburg, St. Maarten (Dutch side) is home to a lot of day-trip catamarans that very often need crew during the winter high season. You might want to look there, too.
Puerto Rico, generally, is an island that most cruisers just try to get through and so don't really congregate the way they do elsewhere in the Caribbean, so you might have less chance of finding a boat there. But you could get lucky. The south coast of Puerto Rico is where the boats would be traveling.