The best thing to do about diving is to wait till you're here, then walk around and check out the dive centers. Choose a real dive center (one that does the diving itself instead of simply selling it on) and you should have no problems. Lots of people have strong preferences, for positive or negative reasons, but to save KE any possible embarrassment let me say that Amigos del Mar is certainly one of the best. If I were a visitor to San Pedro I might well dive with them. But it isn't the only good one.