you anticipate the bank falling off the continent into the ocean, use a ziploc, for pennies or even better, free at airport TSA checkpoints.
I recently purchased a 1.5 TB portable Seagate GoFlex drive to use for off-site backup in a safety deposit box at the bank. One thing I thought may be good for it, in case of a water leak or the bank's sprinkler system goes off, is a water-proof bag or some sort of water-proof padded pouch (if anyone can recommend one, I'm all ears). But my concern with that was... would a water-proof bag be too air tight and create some condensation/moisture inside that would be bad for the drive? Just trying to figure out if I should get one or not before I go on the search for a small one for my drive.