We went through a certified dive shop, http://www.akumaldivecenter.com/index.html and the Dive Master went with us. After reviewing the signals and equipment, we were in a pool for about 45 minutes to practice signals, getting comfortable with breathing, how to find a regulator, adjust the air in your vest, check your air, clearing your mask, equalizing pressure in your ears, and buddy breathing.
Then, we went in a boat to a spot with a flat bottom (with no drop offs). They had a rope anchored to the bottom that we used the rope to help us get down. Our instructor stayed with us the whole time and we checked our air supply a couple of times. At 40 feet, the visibility is clear and the lighting is bright. A big thing on your first dive is trying to maintain/control your bouyancy. I got pretty comfortable with it after a few minutes.
We bought an underwater camera with interchangeable film/flash cartridges at our local Dive Shop. The camera with the first roll was about $20; each additional roll was about $10. The camera was water tight to at least 100 feet. I got the pictures developed at Costco. My only complaint about the camera was that the shutter button was higher than the other controls. Thus, if I left the camera cocked and ready to shoot, I wasted a picture every time the camera button bumped something. Next time, I'll only advance the film frame right before I want the use the camera.
In Akumal, I saw a disposible camera made by Kodak (which I had not seen before). If you can find them here that might be an option; but, don't know how deep you can dive with them. I'll bet they are cheaper here than at a resort.
During our dive, our instructor took a bunch of pictures with a high end underwater digital camera. After the dive, we had the option of buying a CD full of the pictures. Those were the best underwater pictures of all.
In order to get fully certified on our trip, I would have had to have been away from the wife and Boys for 5 whole days. The introductory course was a good way to see if I would like it or not. At the shop I was at, they would have given me credit towards the full course.
My wife liked being underwater; but, didn't like the bobbing up and down on the surface of the ocean. It made her a bit queasy and uncomfortable. Thus, she was not too eager to spend 5 days getting certified.
Best of luck,
Roy