Whereas I appreciate that the article is a brief overview, there are far more things to consider when shooting underwater than the basics here and I would never advise simply learning to dive then jumping in the water with a camera if you want decent results.
I am a professional Underwater Cinematographer and it takes years and a lot of financial investment to gain the training and skill to shoot exceptional underwater images. The first thing is to become an excellent diver before even considering taking a camera with you and this takes a lot of dives in different conditions to achieve, not only that, you also have to consider the safety factors, risk assessments, bail out plans and everything that happens when things go wrong.
My most recent shoot was a behind the scenes film for a 360VR project about Mobula Rays which you can see here https://vimeo.com/185483045 and I'd be happy to answer any questions anyone may have about filming underwater either here or via my website www.scarletviewmedia.com
It's nice to see underwater filming mentioned here as it's not often mentioned.
Keep up the great work with the site!