I'd have to believe that given the choice of Pt Hardy or Gulf Islands, and recognizing your desire to see/photograph marine mammals, Port Hardy is the better choice. The opportunity to dive in proximity to sea lions and seals would be greater near the smaller, unpopulated islands around northern Vancouver Island. If live-aboard isn't an absolute requirement, I'd suggest looking at
God's Pocket as a dive op / living arrangement. With the smaller boat, and an intimate day-to-day knowledge of the local waters (and the usual locations of critters who live in them), they'd be my first choice. Lots of commercial marine traffic around the Gulf Islands (ferries). I've dove some sites around Pt Hardy and have to say it is a photographer's (cold water) paradise. Looking forward to a trip report, regardless of what you pick.
C.