You won't have any problems in San Carlos except transport. Dock at Marina Terra and walk out front and catch a cab to Gary's or El Mar. We rent tanks form them regularly and take them onto a private charter, We all have our own gear but they must have rentals .
I know we've rented from El Mar and I'm pretty sure Gary's also. Some of the other operations are boat based in the marina so likely don't have any gear to rent.El Mar is in AZ but they have a sister shop in San Carlos. I'm sure it's on their website.They also keep a nice dive boat there - Gary's has several.
El Mar Diving Center
Gary's Dive Shop - Scuba Diving, Sea of Cortez, San Carlos, Mexico
Everybody goes to the island - San Pedro Nolassco. On the (west?) side is the sea lion colony - they;re so used to divers they'll be falling off the rocks as you moor. And be waiting for you.
There's also some old wreck out there but IDK where it is and it's supposed to be pretty beat up. I've heard of shore diving off San Carlos also but IDK where.
Way off the south side it gets deeper. I'm not good with distances on the water but the island was pretty tiny from there. There's a canyon there that brings n big stuff.
On the off-chance that you're going next month - the scalloped (smaller) hammerheads school there over a couple days b4 heading south - often the 2nd/3rd week of October. You have to sneak up on them - ask at the shops they'll know when. Not sure if they'll share gps numbers though. They're pretty skittish. It's a guessing game - we missed it by a week once. All gone and my buddy had seen hundreds the week before.The regular dive trips don't go there as it's kind of rough for their newer divers.
It will also be warm in Oct. I dove in a t-shirt about Oct 20th. Below the thermocline at about 60' was too cold to stay down long though.
Also as you traverse between the island and San Carlos watch for the dolphins in the late afternoon
Marina Terra doesn't have a great restaurant though - they cater to bus tours. So it can be you and 100 seniors for early dinner.
I don't remember the name but off about 2 blocks to the left of the marina there's a small plaza with a mexican food place - good food and outdoor patio. Also the restaurant to the south of Marina Terra was a diver's hangout - it may have closed.
San Carlos is an ex-pat retirement spot that has about 5 dive operators since divers from Phoenix and Tucson drive down weekends.
If you haven't seen it -
San Carlos, Sonora, Mexico
I think aztinman here has been there recently - more recent than me .