NAUI (National Association of Underwater Instructors I believe) was a course that I took many years ago.. a pretty extensive one. It was for four hours per week for about 2.5 months. Half in the classroom, half in the pool, followed by a series of dives to get a license to 100ft for level one. I don't know how it compares to the other certifications. However, unless you go through a decent course, I would not recommend going much below 15 to 20 feet, you can do some nasty damage to yourself if you don't know what you are doing. I really don't agree with the resorts doing a half hour lesson followed by a dive for people who have never dived before. But they do take you in pretty shallow water usually. Sadly enough, on my many trips away I have not gone diving (the snorkelling seemed to be very adequate). I do recall that one of my test places was Sherkston Quarry, and it was kinda cool because it was a quarry that filled so fast that they could not remove the equipment. So you are hovering over trains and buildings... almost feelig like you are flying. Dang, all this talk of diving makes me want to get out and do it again.