We've got the question a couple of times - given that the new Yubikey 2.0 is completely monolithic - is it waterproof ?
The answer is yes ! There is however "grades" what really waterproof means. Preliminary tests indicate that we'll at least meet Ingress Protection class IP67, probably even IP68. Salt water is a slightly different game as it can cause corrosion. If the Yubikey is rinsed in water and dried, it should however be fine.
Read more on IP ratings at: http://en.wikipedia.org/wiki/Ingress_protection
We are to test it according to appropriate environmental standards, not only water resistance.
With the best regards,
JakobE
Hardware- and firmware guy @ Yubico