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what I learned on vacation
Take your Droid out of your pocket before scuba diving. It turns out that this can damage certain models of mobile phone and even void your warranty.
I'm referring here specifically to the first version of the Motorola Moto Droid. I have no specific experience with other sorts of mobile phones.
Also, it's a good idea to take your wallet out of your pocket, especially if you have any of those electronic keys that look like big white cards. Something about the environment seems to ruin their ability to open doors. Regular credit cards with the magnetic strip seem to survive immersion down to 100' with no ill effects as long as they are properly rinsed and dried.