Most of the diving on V.I. is on the east coast ... in the waters between V.I. and the mainland. As others have described, the wrecks in Nanaimo are excellent dives ... as are some of the walls and pinnacles such as Snake Island, Jesse Island, and Four Fathom Reef. You can do a search on those terms and find some good site descriptions in this forum.
Quadra Island is as far north as I've yet been ... and as earlier described, has more life-per-square-foot than anyplace I've ever dove (including places in the Caribbean and South Pacific).
Barkley Sound is on the west coast of the island. While it doesn't have the sheer abundance of marine life you'll find at Quadra, it more than makes up for it in variety and color ... in fact, overall I favor it over Quadra Island because it's more protected and therefore you get more opportunities to dive here. It's mostly pinnacles (underwater mountains) who's "peaks" are anywhere from 20 to roughly 60 fsw ... and from there you can plunge the depths to your heart's desire. There are also some great wall dives ... and "fingers" coming off some of the points in the Broken Group Islands that are incredible dive sites. Marine critters here are in some ways quite different from what you'll see on the eastern side of the island. Giant piscavora anemones come in many colors, providing red, orange, yellow, and green hues to a pink and lavender background of hydrocorals. Pale sponges compete for space on the rocks and walls with many species of cold water corals, hydroids, strawberry anemones, zoanthids, and long-needled red and purple urchins. Giant pacific octopus, wolf eels, and Puget Sound king crabs are abundant, as are many species of rockfish, ling cod (I saw one weighing perhaps 150 lb up there in early June), and massive schools of smaller fish species. Six-gill are commonly sighted at many of the pinnacles ... especially during the summer months. And I've seen as many as a dozen different species of nudibranchs on a single dive, including the giant dendronotis (both red and white) swimming nudi's and the gorgeous diversicolor and crassicornis.
I have dove off of Victoria, Nanaimo, Hornby Island, Quadra Island, and Barkley Sound ... and of those, the last is without question my all-time favorite cold-water dive destination.
... Bob (Grateful Diver)