Reefs are the main attraction on St. Vincent with no major walls to boast about unlike its volcanic island counterparts. Deep dives are available however, and with great visibility and pristine reefs no one should be disappointed. With this in mind, sites are relatively uncrowded at present, since the island is not touted as a major dive destination. Look for black coral at shallow depths as well as seahorses.
Bequia in the Grenadines shouldn't be missed either. Here you'll find wall and drift dives.
Other drift dives and a wreck dive are found off of the Tobago Cays and Mayreau where all levels of experience are welcome.