do you think you'll be diving or snorkelling most?
Snorkelling, since it's so shallow, should be fine with the on-camera flash - definitely use it on all your shots and if you can diffuse it with something that would be good.
Diving will be darker so an off-camera flash would be better although this makes the rig a lot more bulky - I'd say for your first time, don't worry about it but it's essentially a must-have if you want to do more. There are plenty that you can attach via an optical sensor to the built-in flash on your compact so don't worry about that - I can point you in the right direction if you want to know more about those.
Camera settings will depend on what you're shooting and whether you have an external flash - if you get one you might want to get into balancing the flash/ambient light like topside strobist shots, otherwise just expose for your subject.
Macro or wide angle are the two most common UW styles although you may struggle with macro with you're snorkelling since it usually takes a while to find interesting stuff. Either way, don't be tempted to shoot anything too far away - your camera may have a zoom on it but you'll be shooting though lots of water with a lot of minuscule stuff floating in it that will reflect in your flash light.
You'll probably want to increase your ISO a fair bit so experiment with how far you can push it and still feel happy with the noise levels.
The deeper you go, the more blue tint you'll see in your shots - snorkelling and using a flash, you may not see much at all, diving you almost certainly will do. Many UW photographers take a white slate down to calibrate the whitebalance at different depths although if you shoot raw this is less of an issue.
Make sure you familiarise yourself with the housing before you go - test it in a bath/sink with some paper inside (you could even test it on your first dive in the same way) to make sure the seals are secure and not leaking water that'll fry your camera.
Always fit the camera somewhere away from dust/sand - the big thing here is to ensure that the O-ring does not have any grit on it and is lightly coated in silicone grease. When you shut the case be careful that you don't trap the o-ring when you shut it.
You may also find that you can't access all of the buttons you usually can so have a play before your first dive!
If you really want to learn more then check out a couple of other forums:
Yorkshire-divers (photography section),
digigreen and
wetpixel
Have fun and show us what you take!
