I am not nuts (on this anyway).
The rubber disc sits on the opening, and the spring pushes down against it. You can blow air out of the diff side with some force to open the valve... as in hot gasses escaping the diff..
However, unless that spring and rubber disc is removed, you cannot blow air into the diff. Blow as hard as you like! The water is NOT getting in here with that disc and spring in place. It gets in past the axle seals.
Putting a 2 way breather in place of this one way one in your picture (up higher at the end of a hose), is what eliminates a vacuum that would cause water to get it. Why does Toyota do this on the front diff., if not true? (of course, why don't they do it on the rear??!!)
How much pressure do you think you generate by blowing in something? You cant move a gauge 1 psi. Yes, I have tried. Regardless, water DOES get in thru it. On every vehicle you drive in the water, it WILL get water in the diff.