Here in the deep south of New Zealand I wear a Switlik drysuit, which is fire resistant & fully water proof. A real asset if I ended up in the sea or lake. Also for an un planned night out in the bush. It can rain up to 8 meters a year in the mountains here.
Under that in the cooler months (down to -5 degrees C ) I wear heated motor cycle under clothes with remote heat controllers. Its a real game changer for warm as toast winter flying. I do run it on an aux battery as quite a large current draw.
I use thermal socks, jeans, t-shirt, sweatshirt and a paraglider suit that's warm and windproof, with gloves and a thermal balaclava under my headgear in the winter I tend to wear jeans and a thin long sleeved shirt in the summer so I don't get sunburned
I was wondering about using a drysuit during winter months. Probably a kayak type drysuit would work as one doesn't need the inflation and exhaust valves on scuba diving drysuits.
I can't speak directly to flying in open cockpits, however scuba diving in the pacific northwest with rebreathers we use the following guidelines.
Above 45F/7C we use medium weight undergarments with electrically heated vests.
Below 45F/7C we use heavy weight or electrically heated undergarments with electrically heated glove lines. Be aware an electrically heated under garment pulls about 110 watts, and electrically heated glove lines pull about 36 watts. Most electrically heated vests pull about 55-60 watts.