What is a waterproof spotting scope?

Waterproof scopes are designed to totally exclude water from the optical tube. They may also be nitrogen purged: filled with dry, inert gas (nitrogen) as an additional barrier to internal moisture to prevent internal fogging. There may be seals and optical windows, too.

The additional feature of rubber armoring is the final level of ruggedness and helps protect your scope from abrasion, damage, or loss of collimation if you accidentally drop it.

However, this waterproofing isn't meant for submerging. If the spotting scope is submerged underwater for longer than a few seconds then water will begin to seep into the spotting scope. 

Updated 12/18/13