Here's some ideas of some slates we use for cavediving, I've been down to 190 feet with some of there slates and they are definitely waterproof. Both in notebook or slate.
http://www.diverite.com/catalog/slates/ac2061
http://www.diverite.com/catalog/slates/ac2064
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Failure is not an option!
USMC Jungle Environmental Survival Training PI 1985