The Canon PowerShot S2 is a fixed lens camera. Are you using it stock or have you added a lens to it?
The camera has an excellent ability for macro shots. Good choice in camera. Although, Blazer88 mentions 2 things you should keep in mind in your future shots.
1. The images (aside from the first) are a tad overexposed. This washes out the color and unless you intend that to be the effect, it doenst show off the corals very well. You usually find your eyes drawn to the overexposed regions of the image.
2. Keep in mind of what your subject (1st pic) is. Try to frame the image so the subject is in an interesting position. Have the rest of the image (background) compliment your subject to bring attention to what you want the viewer to be looking at.
When you look at other photos, try to understand what the photographer was using the surroundings to direct attention to the subject. One method is called "The Rule of Thirds". You can read more about it here.
http://www.silverlight.co.uk/tutorials/compose_expose/thirds.html
This should help start you off on good composition. When you get that down, just do a search on google for "photography composition". You can work on exposure later.
I hope this doesnt come off as pretentious or anything like that. Just trying to offer some constructive critisism to help improve a fellow photographers skills.