If I understand you completely I think area specific would be better. If you want to see what ABC diver is up to you would find photos and info from his local dives as well as his recent vacation, which might get confusing if you were to see photos of tropical fish in California for example. But if you wanted to see who has been diving in Laguna Beach lately......
As for how specific the region should be, I can only speak for So.Cal. Here most divers seem to dive predominantly the same areas over and over, and occasionally make road trips. Perhaps you could break it down by County? San Diego, Orange County, Los Angeles, Ventura, Santa Barbara, Central Coast, and even a separate listing for the Channel Islands.