From the responses you've gotten so far are about 2 different areas. La Paz being on the Baja side and closer to the pacific, while San Carlos is on the mainland side about mid way up. I can't speak about la Paz, but San Carlos does offer great and diverse diving. I tend to wait until sept/October when the air temps start cooling down. In July and August, is monsoon season out here and Laurie S. would probably have a better idea of the conditions then. Water temps will be in the 80's until late October or early November
Vis 50-100 feet. There is an abundance of sea life and critters to see, some of which is unique to the area. As of yet, I have not seen any sharks, sea horses, octopi, or frog fish, but I know they're out there. But I have never had a dive where there wasn't a ton to see. Isla San Pedro Nolasco hosts a colony of Sea Lions which are great if you've never dove with them. The females and pups are playful, just keep your fingers to yourself. Night dives are also a great option, and offer a completely different set of critters to see.
my favorite sites are Southpoint and las cascaditas(little waterfalls) in the Isla San Pedro area, and the Sea Mount or window rock(dependent on conditions) closer to town. In late October, the hammerheads school, but I have yet to make that dive.
We stay at the Playa Blanca Condominiums which are at the higher end of the spectrum, but have a private beach and large pool(why people want a pool is beyond me!?!). They are fully furnished with a kitchen, washer and dryer, whirlpool bath, and every one has terrific balcony views. They're about 5 minutes from the Marina and we generally use El Mar for a diver op. great boat and crew. Sunsets at the Soggy Peso are a must, and there are several great restaurants.
most of us drive down from Tucson, but flights are available into Guaymas, Sonora which is 30minutes away.
Another option would be the Rocio del Mar liveaboard.