I could write a pretty long artical on all the mistakes I have made, but I would have to use a fake name to protect what little credibility I have. This is not an all inclusive list, but....
1) Create the setup checklist for prepping your camera and housing.
2) Do all or most of your setup in your room before heading for the dive. Test fire your strobes in TTL before you head out. Cover the lens and fire. Your strobes should do a full dump. Shoot into a mirror.
3) Make sure film is loaded properly and advancing.
4) Make sure your lens is set to AF if that is what you want. Don't forget the limit switch if your lens has one. Make sure MSC switch is where you want it. Check all knobs, dials and gears are working properly and camera is closed properly.
5) Check metering mode and shooting mode to make sure they are set correctly. Changing them underwater is a pain.
6) This is my most important tip. Keep it simple. Then add variables. Start by turning your camera into a high quality point and shoot. Shooting macro with a 60mm lens, set to aperture priority, f/16 or f/22, strobe on TTL, then focus and blast away.
Now you have no camera adjustments and you can focus on shooting the subject rather than fiddling with your camera. Follow the techniques mentioned by others. Find your subject. Start with subjects that don't move. Decide how to approach to get the shot you want (angle of approach, subject angle, subject position in the frame, negative space-what do you want the non-subject area to look like, and is there a safe spot to place my fingers).
Once you are comfortable with the above, then add one complexity. Adjust your strobe aim. This takes practice. Unless you are using a ring flash, the flash is always coming at an angle. Every change in distance from camera to subject means the angle of light has changed. If I'm shooting a tiny subject right in front of the camera with the strobe aimed properly and I then try to shoot a larger fish, further away, the strobe is now aimed in front of the subject and perhaps below the subject. A slight adjustment takes care of the problem.
Again, eliminate task loading by keeping it simple. If anyone saw my first dive with a housed camera, they would be rolling on the floor laughing. Low vis, current, surge, trying to shoot manual while adjusting EV. None of the six shots I fired were very good. Don't mess with your camera on the boat. Start by shooting with a setup that requires no underwater adjustments.
I am a firm believer that improvement starts with the elimination of camera setting adjustments and concentrating on taking the picture.
And of course, shoot, shoot, shoot!!!! Look at the results. Figure out what worked and didn't work.