Dexshell waterproof socks - the biggest kit discovery I have made in years - get the thick ones, as the thin ones are a bit too fragile. If it is really cold, I wear a thin pair of Injinji toe socks under them. I did try some Sealskin waterproof socks on in a shop, but they were not remotely foot shaped, so I did not buy them.

With Dexshells the waterproof lining will eventually start to leak, but even then you just get damp feet, rather than wet, and your feet remain warm.