Couple of parts to this answer:
- In the Grenadines, there is not anywhere that is far inland, and even on St. Vincent you cannot get more than about 6 miles from the sea.
- Dive shops are almost always at the dock/marina/shore - this may sound obvious, but they are where they will get business: right next to the sea.
Dive shops are on the west coast of Bequiea, in Port Elizabeth; On the south west coast of St Vincent, at the Blue Lagoon, the west coast of Carriacou etc
They all have a good web presence, so as @uncovery mentioned, you can google for "Grenadines dive" or "Grenadines scuba"