I highly recommend getting a pair of these socks.
I used mine today in a race that only had one mud bog, but my feet were nice and dry after the race. My feet really were not cold during the race either. The socks are only double the thickness of a standard bike racing sock.
There are two socks availible. The cheaper ones will protect your feet if the water level does not cross the elastic band. The more expensive ones have a rubber O-Ring seal to prevent any water seepage.
Dry feet are good!