Using the on board flash only, without a stobe, will limit you for sure. Save up and get a lighting package.
Here are the settings I use for my S&S DX5000G, not sure if the 1200's settings will be the same, but maybe this will help:
PUT THE CAMERA IN "AUTO" MODE, activate the macro button for the majority of your shots, this really just tells the camera to focus on the nearest object. Make sure the focus crosshairs have locked in and take the picture as fast as possible after the focus crosshairs have locked without "jerking" the camera.
MENU & SET-UP SETTINGS:
PIC Quality/size: F2592 (next setting down from uncompressed, uncompressed has WAY too long
lag time, shoot "RAW" when you move up to a DSLR)
FOCUS: AF
PHOTOMETRY: MULT
SHARPNESS: NORMAL
EXPOSURE COMP.: -0.3
WHITE BALANCE: CLOUDY
ISO: 100
These settings were given to me to use from the people I purchased the camera from and have worked very well for me, see my galleries for samples.
These cameras strong suite is the macro feature, although once you get a strobe and can light larger areas, you can get some very nice reef shots. Just as a rule of thumb, try to shoot things that are within the distance of an out streched arm. Things further away may be in focus, but you won't get much color.
one thing to remember, if this is your first digital, and I still have problems with it. Keep the camera as still as possible until you see the shot in the review window, moving the camera too soon can cause blurriness and make your pictures appear out of focus.
Keeping the camera still underwater seems easier than it is on land, the water pressure must help with stablization. Practice, practice, practice shooting flowers, bugs, anything small on land in macro mode, this will help train your muscles to holding still.
Have a great trip and best of luck.