on a side note your head looses about 5-10 % of your body heat , the 2/3's of body heat applies to babies due to the fact their head is disproportional to their bodies. your hands / feet / head give off the same amount of heat in adults .
This has been my first winter surfing. I'm in VB, the water is 42, and a 4/3 has worked for me so far. I start feeling cold after a little over an hour, depending on the wind. But I'm over 50. A hood is also a plus when it's windy. Even with a hood, I sometimes have ringing in my ears for an hour or so after I'm out.