Hello BC has a really nice write up about scuba diving in Nanaimo:

Legendary oceanographer Jacques Cousteau once described the Strait of Georgia  as “the best temperate water diving in the world, second only to the Red Sea.” Not surprisingly then, scuba divers eager to forgo a trip to the Middle East are thrilled by the proximity of world-class marine adventures near Nanaimo.

Nanaimo has smartly established itself as the premier diving destination in Canada. The big attraction: Three “artificial reefs” created from decommissioned and scuttled navy ships. A handful of scuba diving outfitters and charter companies can provide all necessary gear to make the experience safe and memorable.

Divers flock here to either explore the ships or introduce themselves to the local marine life from walk-in spots along the coast. The cold, green waters are home to giant Pacific octopi, wolf eels, sea lions, seals and an undersea garden of sponges, kelp beds and brilliantly colored anemones.
Artificial Reefs

The sunken ships are a major draw for thousands of divers annually. Representatives from the Artificial Reef Society of British Columbia and the Cousteau Society were on hand for the sinking of the H.M.C.S. Saskatchewan in 1997. The 366ft/112m destroyer now sits on the ocean floor near Snake Island, a bird sanctuary and seal colony east of Nanaimo.

A few dozen flipper strokes from the Saskatchewan is the H.M.C.S. Cape Breton, a Second World War supply vessel sunk in 2001. Recognized as the world’s largest artificial upright reef, it’s a playground for advanced divers.

Beginners are advised to hone their diving skills on the area’s latest artificial reef. A deep-sea rescue tugboat, commissioned in 1940 for service in the North Sea off the English coast, was sunk in 2005 in the relatively shallow waters of Departure Bay.

Enjoy your scuba adventure!