I imagine it would get pretty wet if you baled out into the sea wearing it, nevertheless, it was a life saver for some airmen............it's the actual wetting that prevents the water from penetrating.
The cotton fibres expand causing any gaps between them to close tightly.
Obviously, you wouldn't wear it in contact with your skin.....a merino layer(s) underneath would be desirable in cold windy conditions, especially if you're climbing or hiking.
Pete