Yeh they do great in mud and sloppy conditions. Yes as NZCaver said.. they are waterproof to the top. The inside is made of the same thing kinda just like a bootie? if ya understand me lol. I think thats why when they are full of water they still keep your feet rather warm compared to other boots. They make so many different types. You should really check them out if you want a great warm waterproof boot. The 16" Wetlands i have are 5mm.
I have the Wetlands also. I have been caving in them for four years now. IMO, they are a superior caving boot.
The only problem I have found with them is that they float. When you swim with them it is hard to keep your feet underwater.