? Oh, cold, hummmm? Yup, a t-shirt, with short sleeves if it's bitter like the low 70's. I think for Christmas we might swim out and decorate the REEF!
Under armor and thermals, then just regular outer clothes, then i just throw on my old military field jacket and go to work. For my head I use this thin layer that leaves only your eyes and nose visible. I find that it traps the body heat from my block head and keeps my noodle warm. I work outside constantly, and find this really works well for me. That and a thermos full of spiked coffee.
I only go detecting in the cold, if the grounds not frozen solid. Don't see the point of trying to extract valuable coins or relics from frozen dirt you will only damage them.
But if you plan on going out then I would recommend the Sealskinze line of thermal clothing, they do Beanie hats, gloves and long socks that are brilliant in the cold