Most Tucson area divers who travel to San Carlos simply use the truck route crossing on the west side of Nogales. It's fast and much more of a direct route than other points of entry/routes indicated in other posts here. The road south from Nogales is a toll road that's relatively safe as well. Twenty bucks (US) at the first south bound toll station will get you enough pesos to pay the toll at the remaining toll booths. Other roads from the border to San Carlos have more potential safety issues (country west of Nogales and Hermosillo can be spooky with potential cartel activity). 72 Aquatics (formerly Desert Divers) out of Tucson has been running trips to San Carlos twice a month for over twenty years without issues. They use the Nogales truck route crossing. It's worth noting, the return trip back up the toll road to Nogales requires tolls to be paid in pesos.
As noted in other posts, water temperatures in March will require a heavy wetsuit, multiple wetsuit layers, or a dry suit. Spring visibility isn't usually the greatest. March is often windy. If the wind is too heavy, the boats will not go out to Isla San Pedro Nolasco; you'll stay at local dive sites.
Again, as noted El Mar has a really good dive boat in San Carlos. Ocean Sports (used to be a sister shop associated with the Tucson Desert Divers Shop) also has a really good boat. Ocean Sports operates independently (Alcantar family in San Carlos) now, but maintains a good working relationship with the Tucson shop. Both of these shops do reliable air fills. Their twin diesel-drive boats are well-maintained and crews are very experienced. If you want to book a slow dive boat, Gary's can help with that.
There is no Best Western in San Carlos. What used to be a Best Western changed hands and names several years ago. If you're with a group that's looking for a place to stay, it's worth looking at a site like Home Away. We use the site when we're looking to rent a condo and there are usually lots of options.
Most of the diving is done at Isla San Pedro Nolasco and some local dive sites. There are a couple of wrecks (a ferry and a Japanese tuna boat) that don't see much action due to depth. The shops typically use the same local sites. A lot of Arizona and Mexican dive shops use San Carlos trips to do open water cert checkouts for new divers. Make sure your group gets booking dates scheduled in advance.
If you do deeper depths off Isla San Pedro Nolasco, it's worth noting that I've been hearing some accounts of divers seeing sharks from time to time. Sightings have been very infrequent. If you happen to see a shark, it will likely be a bull. There are bull sharks that occasionally make appearances living in the deep underwater canyons between the island and mainland. Aggressive behavior hasn't been an issue. White shark sightings are even less frequent than sightings of bulls, but I've heard a few reports. Shark finning has really reduced shark numbers in the Gulf of California.
Gene Foster is still active at El Mar Dive Center. While his daughter Tonya runs the business end of things, Gene makes sure the Mesa and San Carlos shops are maintained. He also plays a role in the regular maintenance of El Mar's dive boat in San Carlos. I enjoy visiting with Gene. He is one of the guys who pioneered scuba diving in San Carlos and has incredible knowledge about the Gulf of California, particularly the Sonora side.
-AZTinman