The small village of Castara in the north of Tobago is blessed with some of the best dives sites on the island, most of them unknown as they are considered too far away by most diveshops. The reefs range from the gentle sloping, to the steep drop offs, as well as deep underwater pinnacles. Both, beach and boat dives are possible here. 
Castara is also just a 20 minute boat ride from the famous dive site "The Sisters", as well as a few other great sites like "Big Market", which is known for having large pelagics.