In the event you end up shivering uncontrollably underwater, it is time to end the dive, surface and warm up.
You can also help slow down the process of getting cold by protecting your body temperature pre and post dive. A simple way to minimise heat loss is to wear a hat when you are on the surface. A humble beanie works well, and there is quite a choice out there. The latest one to hit the shelves is a limited edition 'seasonal beanie' from O'Three.