Some disposable water cameras also have a color adjustment for underwater photos (don't know if it's the film type or part of the lens), so they can give an odd color-cast for photos taken out of the water. I don't know if this is true of all models or of some.
However, I agree - protect your digital at all costs!! I want to take some photos on Grizlly River Rapids in California Adventure and I'm considering buying a disposable for that (but not the waterproof beause of the color issues). If water gets in and ruins it, at least I'm only out a few bucks.
If you go to your digital camera'a manufacturer's website, you might find they have an underwtaer case as an accessory. I know this is true for the Canon Powershot series. I've thought about that as well - a little more epxensive (but not much more if you use 5-7 disposable underwater cameras during the life of your digital).
Dirk