I wear polypro long undies, then fleece, then neoprene for super cold rivers and breathables for everything else. You can beef up the fleece and go breathable or lighten up and go neoprene.
Socks that are designed for performance use (hiking, climbing, skiing) are important. I buy at outdoor equipment stores and pay the $10-15 / pair, so very worth it on a winter river. Formed to fit very nicely and padded in all the right places.
Shoes that are too tight are the worst thing for cold feet, choosing the right performance sock that is not too thick or layering with high quality materials is the key IMHO.
On top I swear by the Techwick medium weight zip-front longsleeve mockturtleneck pullover. It can be worn under a comfortable shirt & fleece jacket under an all-around shell like goretex. Freedom of movement, lightweight, cools quickly after walking opened up and keeps you toasty when zipped up.
You lose a lot of heat form your noggin, so a good headcover is critical. Gloves are a whole nuther topic too.
When it comes to performance clothing, it pays to buy high-end IMHO. It lasts a long time and it's easy to appreciate in the conditions it gets used in.