Under Water Photography Guide - https://www.uwphotographyguide.com/ has a ton of really good information, especially for beginners. I recommend everyone check it out regardless of skill level.
UWPMAy puts out an extensive pdf every two months at http://www.uwpmag.com/ . It's a large pdf to download (usually 20-25mb) so it takes a minute to load. You also have to sign up with an email to get it, but I've never been spammed by them over the years. I've attached the most recent file.