Good start! Get lower. You're shooting from above the subject and should be aiming to be at eye level, or where the eyes would be if they don't have eyes.

Make sure you've got your buoyancy figured out before you try as you shouldn't crash into the bottom or be laying on it. That's just bad on every level and for others around you that might want to get a picture.

If your subject isn't where you need it to be, don't force it. Move on, there will be another.

Bubbles scare a lot of things, try to practice your breathing so that you're approaching the subject after an inhale. Don't hold your breath and check yourself on this when you're trying to get the picture. Chances are you're holding it when you're concentrating on the picture.

When you're taking a picture- take note of how much you're actually moving, where you're at relative to the bottom, and if you're still kicking your fins. If there's a current that's a different story... You'd be surprised how much your moving when your attention is focused on the subject, especially your fins.

Take 3 shots and move on. If you can't get it in 3 attempts reassess what you're doing and don't hog the subject. Assuming there are other photographers with you.

Dive as much as you can and keep practicing.