Manual manual manual - learn how it works and why...it isn't too hard, just takes a little practice! You'll do great.
Practice with the camera in the housing before you head off on holidays/diving. Know thy buttons!
One word about getting too close - remember that the camera has a minimum focussing distance, too. This is how far you have to be away from your subject for it to be in focus - the Canons are pretty good in the regard, but, again, practice on land to get a feel for that zone!
Threads on any of the Canon A series will help you and threads on any getting started techniques for any camera system will also be applicable more or less - you'll just need to tailor for your camera.
have fun!