I've dove a 7mm 2 piece suit (farmer john and jacket), in water down to 36F, for up to 40 minutes. I finally did break down and get a drysuit (and actually use it - sometimes), not because of the temps, but for those days when the cold surface conditions make it impossible to get in multiple dives in the wetsuit. So, for 19* air temps - it's dry suit time! If the water is 36, but air is 60 - wetsuit.
Ken
Ken