2 reasons. First, this book:
Mysterious Undersea World: Jan Leslie Cook: 9780870443220: Amazon.com: Books
Second, The Undersea World of Jaques Cousteau.
I'm not sure how many times I read through that book but I knew by the time I was 12 that I needed to do this. I grew up not far Harvey's Lake in Northeast PA, and at the time (early 80's) an old salt ex-Navy guy had a small dive shop there. I begged and pleaded daily for my parents to take me out there. I forget the cost but you could basically go out there and rent gear and after 5-10 minutes of instruction, swim along the shoreline. He also had a couple boats sunk in 20-25 feet that he would take you to if he thought you were comfortable. I was hooked after day 1.
The next summer we planned a family vacation to the Keys. We knew that there would be some sort of class I'd have to take to be able to dive there, but that's about all we knew. Tommy (owner) told us that for $100 he'd certify me. So basically that summer from June - August, I was at the lake almost every day diving 2-3 tanks per day. All for $100. He taught me about BC's (horse collar at the time), reg clears, mask clears, emergency ascents from about 30 ft. But most of my diving was solo. I dove doubles, I dove the old corrugated 2-hose regs, I was a tour guide for much older first-timers... And my classroom consisted of my flipping through a huge binder of navy dive tables. Unacceptable by today's standards but I learned more that summer than a lot of people in their entire diving lifetimes. My certification card was a typed out, laminated card from "Harvey's Lake Dive School". No affiliation whatsoever.
Needless to say when we got to the keys, the first shop looked at me like I had 3 eyes when I presented that card. I ended up finding a shop that agreed to take me out, and they couldn't believe how comfortable I was. By now though my parents realized that this cert wasn't going to cut it, so I ended up doing a PADI OW class on that trip. Since then I've gone on to AOW, did my rescue cert while living in San Antonio in the late 90's, my DM Cert while living in San Francisco/diving in Monterey, early 2000's, and now some 35ish years after first diving, am wrapping up an Asst. Instructor cert with plans to go right into full instructor.