Well I don't know if they're supposed to be but our pasture has been really wet and I was taking hay out there the other day in mine and just about got my boot sucked off in the mud and my foot stayed dry.
The ones that are waterproof have "Waterproof" embossed into the leather on the sides of the heels. I bought my kiddo some for Christmas and just looked at them.
That being said, I have the non-waterproof ones and while I don't go wading through puddles on purpose or anything of the sort, my feet have never gotten wet while walking through rain, snow, slush and the like.
The waterproof ones are shinier, they definitely have a coating of some variety on the leather to the point where my hubby thought they were "pleather" boots at first.