I'll second the vote for Cabo Pulmo- it's just a bit of a drive from Cabo and when I was there it was still dirt road for the last third or the ride. Still, it's one of the nicer dive sites down there.
Another place to check out, closer to town, is Chileno beach. It's not that far of a drive, and has a dive shop on the beach where you can rent gear and kayaks. It is also fairly protected but still allows you to work your way out into deeper water. I've dove it this time of year before and saw lots of life.
Since you'll be in town you can always take a water taxi out to the arch and snorkel off the Pelican Rock area- just watch out for the jetskiers.
Jon
