Advanced open water here.
Vancouver is actually one of the nicest places in the world to do scuba diving believe it or not. The eco-diversity makes for some awesome dives as long as you have a dry-suit. Waters are too cold for a wetsuit and man, it would just suck.
Greenstoner:
http://www.divinglocker.ca/ is where you want to head to -> get your basic PADI and if you can, do the drysuit certification.
Porteau Cove and Whytecliff parks are some of the closer dive sites. The shipwrecks are pretty neat to explore.
Best time to dive is actually the winter because the water is more clear, but without a drysuit, it would really really suck.