I am completely new to u/w photography, and diving for that matter. I have just recently become scuba certified, and love it, but you all know the feeling.
I do not want to be negative here, but as I read your post you just startet to do scuba diving? If this is the case I would suggest you first aquire some diving skills. Taking pictures under water requires quite a bit of diving skills, which you will not get over night. You must have excellent control of your buoyancy, not only floating horizontally but also on your back and maybe upside down. In other words, you should reach a skill level, where you do not think about all that anymore. Otherwise you will not be able to handel additional equipment (such as a D-SLR rig) and focus on the picture itself.