If the seal was directly exposed to the water pressure AND it didn't have the outer lip seal AND it was stationary for a long period of time that may happen. Not gonna happen driving thru puddles, not gonna happen even if the water is deep enough to go over the axle and you blow thru it. All the water pressure is displaced by the axle punching thru the water and it does not have time to hit the seals.
Really guys, dropping a hot gear box into water and using that as a definitve example that water gets by the seals? Comparing ATV's with a totally different sealing system? :-laf Ya gotta make some realistic comparisons here.
The axle seals are buried behind the drums and there is no possible way to put the force of water to them. The pinion seal has a cover that completely protects it from direct contact. The only pressure applied to the seals is indirect and very little if one is moving. The outer lip seal is both a dust and water seal. It is formed to seal tighter with more pressure. Again, if the correct seals are used and they are good no water is gonna get by them.
I spent many years fording rivers, creeks and standing water with all kinds of 4x4's. Seldom had water in the diff problems. It just doesn't happen. The only time we had problems was if the breather tube was cracked or tore off, or, we stuck one and had to let it sit for an exteneded time in water.
Condensation cannot make the amount needed to destroy the gears, just driving in rain cannot let in that amount of moisture. The OP doesn't launch a boat and unless he is a closet mud bogger that leaves the truck sitting overnight stuck somewhere, the only possible explanation is the gear oil.
While some of the Lucas products may be good, multiple examples of the gear oil being useless convinces me it ain't.