The Admiral and I went out for a motor cruise last week... sunny day, no wind and 48 F. Towards the end of our 4 hour outing, we were both getting cold (sun was going down fast as was the temperature). We both had on long john's and sweaters and med weight jackets. The Admiral is not fond of going out winter sailing due to the cold. I have to agree with her, add 10-15k of wind on to 40F temps and it's going to be cold!
Ski pants are often waterproof and well insulated and work great. That is what I have non-sailing friends bring when we go sailing in the winter.
If you don't already have ski pants of some flavor and can't find a used pair at Goodwill or Second Ascent then these pants from Fisheries are very nice and what I use:
Gage Burning Daylight Pant
When it is cold (high in the lower 40s or colder) I'll wear heavy (300g/m^2) wool long johns underneath. When it is a bit warmer I'll wear lighter wool long johns.
Up above I just wear layers with a Goretex or eVent shell on top. My sailing "go bag" has an extra layer of wool long johns top and bottom and multiple pairs of gloves.
I keep multiple pairs of these gloves:
Insulated PVC Vinyl Glove - Gloves | Fisheries Supply
and these gloves:
On the boat and in the sailing bag. The second pair are a bit warmer and have better dexterity, but will eventually wet through if it is raining. The former are dry in all conditions.
I keep older "worn out" soft shells, fleece and rain jackets on the boat in a variety of sizes as loaner gear.
A hat and warm boots are other keys. If you keep your hands, feet, and head warm then your torso will also stay warm.
My boat also has cabin heat (Webasto forced hot air) and I'll turn that on when it is really cold so that someone can hang out below and warm up if necessary.