If you want to see the best that Tobago has to offer, you want to be up in the Speyside area in the northeast. Here you get some of the fastest drift dives in the Caribbean, and plenty of marine life to look at, including what is reckoned to be the world's largest brain coral.
You've got Blue Waters Inn, with associated PADI five-star centre Aquamarine, which is a nice resort in a quite, secluded bay, but only a 10-minute walk over a hill to the town of Speyside, which has a couple of rustic bars and restaurants (you've got to eat at Jemma's, which is a treehouse and serves some fab local food).
You've also got Manta Lodge right in Speyside itself, which has its own dive centre.
Most dives are within 10 minutes, and the boats are pirogue-style units designed to handle the swell, etc. Backward roll entry, little ladder to climb back on board.
There is the Maverick wreck down south, but it isn't all that hot, though I would look to do a trip to The Sisters on the western side of the island, it is usually the haunt of a shoal of hammerheads.
Peter Hughes is also running Wave Dancer around the island now, if you fancied a liveaboard, but I'd do the land-based option.
Mark