Hey Tom,
There is a lot of variety of diving within the four main areas, that is Cabo San Lucas (local cabo dives), San Jose Del Cabo (the corridor and Gordo Banks), Cabo Pulmo and La Paz. If you can I would try and dive them all as they are unique and it is easy to get around if you rent a car.
Reefs here in Baja California Sur are much different than the reefs in the Caribbean. Instead of being dominated by reef building corals which actually create the physical structure of the reef the corals simply grow where they can adhere to the surfaces of the rocks. There are some exceptions where stony corals do dominate some of the dive sites but all in all you are looking at dive sites with larger schools of fish and bigger animals such as sea lions, a lot of aquatic bird life, whales, whale sharks and by far the most rays I have seen anywhere.
Definitely a refreshing change if you have dove a lot in the Caribbean.
Some highlights not to miss
Gordo Banks - big pelagic life on some deeper seamounts
Whale sharks in la Paz - 100% guaranteed sightings, I have swam with 12 different individuals in one morning.
El Bajo Seamounts in La Paz have semi-regular sightings of scalloped hammerheads in the fall months
Cabo Pulmo -a unique micro coral reef system and conservation based diving community where no fishing is allowed.
whale watching in Cabo San lucas - not to miss and quite cheap for a half day tour.
I wouldn't worry about booking too far ahead, I find it's best to speak with the people who will actually take you out to get a feel for the dive staff as diving staff does rotate out quite regularly in the diving community.
One piece of advice, bring your own equipment, not all dive shops in Mexico keep their rental equipment in good working order.