It is certainly airtight, and I doubt water would get inside. It does not have an o-ring, so I wouldn't submerge the thing, but you could certainly make an o-ring if you needed to.
Yes, this bear canister is waterproof.
Be advised - the Bear Vault is NOT airtight, and it is NOT waterproof! Those who state that it is are incorrect. There is no O-ring to seal the lid. It is plastic on plastic. Bears CAN find the Bear Vault by smelling your food inside. To help avoid this, place an odor proof bag inside and seal it well. That's what I do. Also, the design of the lid makes it rain tight provided the container remains upright. However if you submerge it, water WILL get inside. It is a good product because of the bear resistant closure, but to be extra safe make sure to use an odor proof bag - not a standard Ziploc or Baggie, but a true odor proof bag designed to meet ASTM standards for air permeability.
The product description says this product is rain proof, so it is probably good for burying something.