From the isn't that a bite department:

Is it a good idea to have your teeth whitened on a cruise ship? Not necessarily, according to an article in the (London) Times Online. It recounts one woman's experience, which turned out not to be a good one. Going into a little more depth is an article on the Cosmetic Surgery Answers website. The bottom line in these articles is that the woman in question hasn't had a good experience, and the experts (who are not really unbiased) quoted in these articles don't think it's a good idea to have the procedures done by anyone other than a trained dentist. Probably the best course of action would be to do some research and consider the possibilities before getting on the ship, and not buying a service such as this on the spur of the moment.