Take a course or workshop on underwater photography with Oceans Below. We have 3 and 5 day courses and you can learn with your own camera or try out some of ours.
Read your camera manual carefully and figure out how to white balance, change settings and focus manually. And then see if you can do all that from your housing too.
Follow some good photographers on Instagram and analyze their photos. What light are they using, which settings, etc. Oftentimes, photographers will add the settings to a post. Otherwise don’t be shy to ask.
Read articles about underwater photography online. There is so much out there!
And then there is Dive, practice and shoot!!!