I'm not sure if I'm proud or embarrassed by this, but I bought a book and taught myself how to dive in 1971 when I was 21 and indestructible. I was also learning to fly airplanes then, and could afford one but not both. I chose flying. I made two or three OW dives with my (certified) buddy in Pennykamp State Park off Key Largo using doubles and a two-hose single-stage regulator. Somehow, I survived. I still have that book, too!
I didn't start officially until I was 51 in December of 2001. Funny--I haven't flown a plane in 20 years. I'm addicted to diving.