Purchase a cryptosteel[1] if you want something that's waterproof and pretty much fire proof.
There's a YouTube video[2] of a dude that tested the cryptosteel by trying to burn it, smashing it, and stuff. It just might help you decide.

[1] https://cryptosteel.com/
[2] https://www.youtube.com/watch?v=u1hLFsBuDT0