Or a sticky note on the bottom of your iPad
Seriously, though, a safety deposit box? It's the 21st century....get a password manager. For example, I've been using LastPass for quite awhile. In addition to keeping my passwords for various sites I am a member of, I can manually add things like the password to my phone or iPad (so if I ever forget it, I can go to my password vault in a browser and look it up). Of course, it does require you remembering the password to your vault....so make sure you put that on a sticky note attached to your monitor (seriously...don't do that).