Most small digital cameras have an underwater housing you can buy which allows you to take your digital camera underwater and get amazing pictures. They are not cheap but IMHO I think well worth it for the memories. Plus, you can see how the pictures turn out before you leave.
Check the website for the camera you own. You will probably have to buy it online, not many stores stock them because there's just too many models out there.
I took this picture a few feet under the water and about 25 feet above this turtle.