Some Caribbean islands are easier to get to than others. Those would be the ones you can fly into. However, once you land think about taking local ferries to experience some of the smaller islands. The following is a short guide to the possibilities:
The USVI and The BVI are well connected to each other, as well as to their local island communities. Ferry companies, Smiths and Speedy's connect Tortola to Virgin Gorda, Jost Van Dyke and other smaller isles in the BVI. St. Thomas and St John are almost joined at the hip due to excellent, frequent service. Tip: You can go from the US side to the British side easily. (http://bvivacations.com/Ferry.html).
There is scheduled service connecting all of St. Vincent's Grenadine island chain from Kingstown in St. Vincent through the islands of Bequia, Canouan, Mayreau, and Union Island. However, the most southerly island Carriacou, belonging to Grenada, can be reached through the mainland of Grenada (www.ospreylines.com), as well as an unreliable weekly cargo boat from Union Island.
Ferries regularly ply the short distance between Marigot in St. Martin to Anguilla (www.link.ai), It is also possible to get over to St. Bart's from St. Martin (www.st-barths.com/guidepgs/public_sea.html).
The Puerto Rican islands of Vieques and Culebra (sometimes referred to as the Spanish Virgins) (www.elenas-vieques.com/ferry.html#vferry) are well connected to the port of Fajardo on the east coast of Puerto Rico.
The Bahamas (www.bahamasferries.com/about_us/about_us.html) is also well serviced by fast ferry from Nassau to a variety of out islands: Eleuthera, Spanish Wells, Harbour Island, Abacos, Cat Island and more. It is a 2 1/2 hour journey from Nassau to Harbour Island.
Tobago has a car ferry from Trinidad, however it's a long trip and there are inexpensive flights connecting these two islands.
Brudy Brothers Ferry (www.brudey-freres.fr) offers scheduled ferry service that connects Guadeloupe, Dominica, Martinique and St. Lucia.