[Christ]

I think the reason for the neoprene being such a good insulator has more to do with it being semi-impermeable when wet.

When you dive, your skin gets wet, but the film of water doesn't move, and your body heat warms the water, which then acts as an insulator. This is why in cold climates and waters, you want to move more slowly to stay warm under the water. The faster you move, the more of the warm water you force out and induce influx of cold water against your skin. If you move more slowly, this happens less.

This comes from one of my uncles who was a Navy boat captain and has a few hundred dives on his record.