I don't know anything about Gary's. It may be good, I've just never had a reason to go to a dive shop in Guaymas. Desert divers in Tucson seems to have a good rep though and they may be a good place to start. Also in Phoenix (where I live) there is a really good little shop that books a lot of trips to San Carlos. It's called Aqua Illusions and I think they have a web site. I like it because it's the kind of shop where they don't try to make you feel stupid if you didn't get your training with them and they don't try to pressure you on anything.
I think Baja Ex only does trips on the Baja side of the gulf.
One idea is to check Orbitz.com or Hotwire.com and look for their deals for trips that include air and hotels to Guaymas. The club med there has some very nice day dive boats and is not too resorty. Only problem is that it's also not very mexican. But you can always go just tour the town.
Also check for packages to La Paz or more likely Cabo San Lucas.
March is unfortunately not the best month for diving in the gulf. there are lots of plankton blooms that are good for fish and whales but not so great for visibility. Still, I've had some good low viz dives there. But then again, I learned to dive in the Mekong river.
If you just gotta go diving in march you might want to check package prices to Cancun, or better yet to Akumal on the Caribbean coast. Very clear and warm.
Good luck with your move to Tucson. It's my home and I spend a lot of time there but it's grown so much I don't know half of it anymore.
If you still need rehab when you get there, a friend of mine is a great physiatrist.
