Most should have some protection, when they spread the oil around while preening gives some protection from lite rain,, some rumples chooks may not have the oil gland at the tail?
This is a trick i use on my gamebirds
For game birds i mist them with lite fine water for about two weeks(once or twice a week) in there brooder before i expose them to the elements, what this does its gets them preening and once they start preening, the oil gland is activated in a sense.
by the time they get outside they will have some mild protection from getting drenched to quickly along with the shelter protection.
Thy giving them a bath--you'll find that it is just as diffiscult to get silkie feathers wet as it is to get any other chickens' feathers wet.
I believe Wild of Pa is most correct and complete in his/her answer, although araucana do indeed have an oil gland.
NYReds is correct that none are waterproof.
Have to agree with you on the silkies. One hot day I got a shallow tub of cold water to soak their feet in as to cool them down. When I set them in, it was so fun to watch as the feathers stuck to each other when going under water, but when brought back up, would fluff out again because of the oils. Hard to explain, but very cool to see.