I am a non-swimmer. My foot cramp in cold water. But I am going to Cozumel vacation next week, really want to try. Anyone tried to scuba dive there? Any advises? Also want to know more about scuba diving before I go, and how to deal with foot cramp while underwater.

Good for you. Coz is an excellent area and one of the few warm water areas I’ll ever go back to. There is some shore diving that can be done, but most of the reefs ( an extension of the 2nd largest barrier reef in the world) are a boat ride out and it’s all drift diving. I’ve found (3 trips there) that the dive shop operators I dealt with were all great outfits (no cattle boats) and I watched the resort course being offered in our resort’s pool. These guys actually took a good deal of time to help the students, unlike many resort courses.
Many of the reefs off the island are in very shallow water, so there will plenty for a novice diver to see and do. Beg, borrow or steal a proper underwater camera (not one of those disposables). You’ll be swimming with turtles, Nurse sharks (harmless but big), Octos, Lobster everywhere and if you’re observant, the odd sea horse not to mention some of the most colourful corals on the planet. A total meal of the colour palate.
Most of the best resorts and diving are on the south west area of the island, so if you’re staying there, great. If not, many of the dive boats leave from the town pier in San Miguel. In San Miguel proper, there’s quite a bit to do. The town gets very busy when the cruise ships are in, so if you plan on hitting Carlos and Charlies (night spot and most popular) or the Hard Rock (just down the street) do it early to get a seat. It quickly gets jammed. The same goes for the market in the town square. Beware of the fake goods some of the kids will try to hawk. Cuban cigars are a popular counterfeit item and if you’re American, with no experience in purchasing them, you’ll easily be fooled. Same goes for other itmes. I was offered a Rolex for 60 bucks, by a kid, when asked if it was real, he said, yes, 95% genuine, lol, I bought a pair of shades off him for his honesty.
There’s a ring road around the island and car rentals are fairly cheap. DON’T rent a moped. It’s not woth the risk and don’t bother renting a jeep, get a "punch buggy" (old style VW Bug with a sunroof). Cheapest and safest. There’s a number of ruins on the island that are worth a visit and you’ll need a taxi or car to get to them. The beaches on the east side of the island are nice but very rugged and not at all sheltered so can be pretty rough. Boogie boarding is popular over there, swimming isn’t.
The water temperature is very comfortable at this time of year and at most a wetsuit shortie is all that’s required. Everyone is different in their cold tolerance.
As for your cramping issue. The water temperature being warm, will be a bonus, but if you do tend to cramp, do a little stretching before the dive and if you’re caught in a cramp mid dive, there’s a technique called a fin stretch, that we do, to get the kink out. Just stop what you’re doing and grab your fin tip, gently pulling back. Don’t worry about your group leaving, while you’re stretching it out.. It’s ALL drift diving there. There’s very little kicking to be done. They won’t be pulling away, you’ll all be drifting in a group.
Hoist one for me in Carlos N Charlies.