If you're wading around in mud which comes up to your knees like I was last weekend (on the top of Cut Gate in the peak district), there's not a lot which will stop the water going down into your boots and soaking your feet, even the seal skins. If I'd have managed to only have to wade up to my ankles, then the seal skins would have done the job fine. I like seal skins - they're the best waterproof socks out there, but they have their limits.