There isn't much online sadly, I was going to start a site based on tutorials and learning but just not had the time.
I can recommend this book highly though http://www.amazon.co.uk/Glass-Water-Freediving-Underwater-Photography/dp/1909455105/ref=sr_1_1?ie=UTF8&qid=1448466737&sr=8-1&keywords=freediving+photography written and photographed by Mark Harris and Laura Storm of the UK who are both adept freedivers and photographers.
Glad you like it, give it a shot and you never know what will happen! If you ever have any other questions, send me a PM and i'll see what I can do.