And indeed such vents are not uncommon at all. It is simply a non-return valve (often a ball type) that allows mist/vapour from the gear box to escape but does not allow any liquids from outside to enter..... unless and until the airbreather is of the highest quality with some technology made specially for amphibious vehicles.(I am sure someone would have designed an air breather for amphibious vehicles too,not done the research yet lazy chap here)
I'm trying to figure out what these number of reasons could be. Can't think of any....Water can enter gearbox due to a number of reasons. ....