A couple easy tips to keep in mind to help with a few of the biggest problems I see in beginner photography...
SHUTTER SPEED - A slow shutter speed means you need to use a tripod and have a slow moving or stationary subject. So if you're shooting a fast moving fish, or fast moving car, or fast moving kids (you get the idea) then you need to have a fast shutter speed to freeze that motion. Usually when shooting fish, I have at LEAST a 1/125th second shutter speed, preferably 1/250th or higher.
TRIPOD - The vast majority of the good coral pictures you see on here were taken with a tripod. This allows you to set the settings to whatever you want and still get the correct exposure. For instance, you could lower your ISO (light sensitivity) down a lot so the picture isnt grainy, which would usually in turn mean you have to lower your shutter speed also to let in more light which would be a problem if you're not on a tripod. But since you're on a tripod, something like a 1/2 second shutter speed isnt an issue for shooting a stationary coral.
PUMPS - Turn them off for the best results. Let your corals and water settle down and stop moving for 5 minutes, then take your pictures. This will help tremendously plus all the little specs in the water either raise to the top or fall to the bottom, helping your shot look cleaner. DONT FORGET TO TURN THEM BACK ON
And lastly, just experiment. With thousands and thousands of shots being possible with no expense other than charging a battery and running a PC to view them up close, you can take as many shots as you want and learn what the different functions do.