Instead of geocaching, I think a treasure hunt type event would be more fun and would be a great business decision for a dive company/shop.
La Jolla Cove would be a really good location for this. Hide 3 little treasure chests underwater the night before. Put a GoPro in one, a card for 20 free airfills at the local shop in one, and a restaurant gift card in one. Advertise it all over social media and at the local dive shops and you'd get a great turnout. Then the company could do a BBQ or something at the park up the stairs for everyone finishing their dives (and people walking by).
It would be a fun event for divers and would have a great turnout (giving back to the community), would allow for great brand advertising, would let you look giving and community orientated all over social media, and would let you advertise to uncertified, potential divers stopping by for a free burger.