Well, I'm not on the mainland (I'm in Port Alberni) but I've done a bit of diving there. Halfmoon bay is pretty good, there's a wreck out in the bay. There's a couple dives worth doing out in Horseshoe bay too but I can't remember the names off the top of my head.
If you make it out to the island there's a couple really good dives just north of Nanaimo in Nanoose Bay. Dolphin beach, Madrona Point and Tyee cove are all really good. Lot's of big octopus, wolf eels, big ling cod and such.