Northeast of Union Island in the Grenadines, the Tobago Cays Marine Park consists of five uninhabited cays and the spectacular Horseshoe Reef surrounding most of them. The best snorkeling in Tobago Cays Marine Park is behind Horseshoe Reef, which protects four of the islands in the calm waters, especially, on either side of Dinghy Pass cutting through the reef. Outside of Horseshoe Reef, there is one other island Petit Tabac that has a small reef of its own worth snorkeling. When passing over deeper areas, keep a look out for nurse sharks and eagle rays.
There are many chartered boat services providing snorkeling access to Tobago Cays Marine Park. The tour operators on Union Island and even St. Vincent offer day trips here, usually combined with a stop at nearby Mayreau Island. Some snorkelers arrive by private or chartered yachts. Keep in mind that it can get busy here in high season to the point where food and souvenir vendors arrive in small motorboats.
To reach St Vincent and the Grenadines by air starting in 2015, you can fly into the new Argyle International Airport on St Vincent which will be offering nonstop flights to both North America and Europe. Alternately, there are inter-island flights from one of the near by Caribbean islands. St Vincent and Union Island are served by inter-island flights from Puerto Rico, St Lucia and Barbados. There is an inter-island ferry service and small inter-island flight service between St Vincent, Union Island and the Grenadines.
Overall, Tobago Cays Marine Park is the 2nd most popular snorkel dive spot of all 7 snorkeling dives in St Vincent.