I've tried the disposable underwater cameras and have always been disappointed with the results. This year I bought an underwater plastic casing that was specifically designed for my digital camera (check your manufacturers website to see if one is available for your camera...most do).
I took a lot of pictures when we went snorkeling in St. Thomas. My pictures turned out so amazing. I have a few of some sea turtles and if I didn't tell you that they were 20-30 feet below us, you would think that I took them on land standing about 2 feet above them. Same vivid beauty with the stingray, starfish and coral reef/fish. The clarity that you get from your digital camera far exceeds any disposable. Some of our family members took pictures with their disposable cameras. They can't believe the difference between theirs and mine.
Let me know if you'd like to see any of my "underwater pictures"