if you want to scuba dive during tour trip to the islands, you can do it at Union and Mayreau with Glenroy Adams Grenadines Dive shop and Bequia with Dive Bequia Bequia shop. Another place to scuba dive is Carriacou with Arawak dive shop.
Critter Capital of the Caribbean I've dived on many places like Barbados, Mexico, etc, and I have to say that St Vincent islands is a very underrated diving spot. Because of that, you will see very few divers, which is great. The reefs, corals and sponges are all pristine. As a matter of fact, St Vincent is known as the “Critter Capital of the Caribbean†and with good cause. It is a macro photographer’s paradise.
If my memory serves me well, the only place that doesn't have dive shops is Mayreau. All the other major islands have dive shops though. If you plan to dive in the Tobago Cays and surrounding islets, go to Grenadines Dive; while in St. Vincent, there's Dive St. Vincent.