I think that would be a great question to ask the bank manager, or whoever is renting you the SD box.
I know mine has a plastic water resistant box inside the actual steel frame. I believe there may be a gasket on the door itself.
Wouldn't matter in Katrina conditons, but in a wind event, I am guessing the safety deposit box wall has lockdown, sprinklers, video survalance and possibly other redundant safeguards not made pulic.
Bottom line, it is a strong box several levels harder than anything in my home.
BTW, When I signed up for mine, I (a white middle aged guy from the burbs) told the banker I was looking for a safe place to "crib my bling"... the guy never cracked a smile and showed me a bunch of the safeguards in place, while my wife stood off to the side rolling her eyes and shaking her head at her idiot husband... As she says, "Men are idiots and I married their king!"