I think it depends on surface temps and air temps as well.
The lakes by me are always 4C on the bottom, but in the suffer time the surface warms up enough to dive in a 3mm jumpsuit. You dive down into the cold for a couple minutes at most and then get to go back up and warm up again.
The problems come in when the surface temps drop and your cold all the time. This means thicker suits and bigger weightbelts. It's the thicker suits and bigger belts that cause the probelm more than the water temperature.
Vis can also effect things. I have had scuba students back out from courses because they didn't believe people actually dove in our local waters- they were certified on holiday where it was warm and clear and became quite upset when I was going to make them dive in water with less than 2 meters of vis.
Jon
