All good points above.
I fish year round here in MA and it can get pretty chilly. My setup:
Socks: Polypro liner socks, thin merino wool socks, and then 100 weght fleece socks. I have found that if I take the insole out of my wading boots, there is plenty of room for that setup.
I use breathables year-round so underneath I go with the following: Spandex polypro thermals under 300 wt fleece pants. An additional layer of wool long johns if neccessary.
Under the wading jacket I sport a polypro liner, wool sweater and then a heavy fleece jacket. I find that the combination of fleece and merino wool is less bulky and warmer than 2 layers of one or the other.
Heavy duty ski hat is key, along with some fleece fingerless gloves and those little handwarmer packets if it really gets bad.
I generally buy all of this stuff for skiing but it works great for foul-weather fishing too!