For safekeeping you're probably better off just using paper wallets. Or if paper is too unreliable medium to store information for you, you can use cryptosteel.com
oh they are a thing of beauty, although knowing my clumsy hands i would end up dropping it overboard like i did with my paperwallet
Then powder coat or galvanize it so it doesn't corrode to much while your retrieving it from the depths.
They are made of stainless steel. The do not corrode. They are pretty damn well military grade. I have one. Worth it IMHO even though expensive.