Hello,
I live in Helsinki, Finland, and for over half the year I am diving under ice, and at a depth over 15 meters, the water temperature is a constant 4 degrees C. Cold water diving has a lot of advantages and disadvantages. The issue is not the water, but whether or not you are cold in the water. Proper thermal protection is the key to any cold water dive. Without it, you can suffer from Hypothermia which can be a life threatening infliction. If your body is cold, you also suffer from a serious degredation in decompression efficiency. The body basically redirects the heating capacity to the core of your body (chest cavity) this is why the extremeties (toes and fingers) are the first to suffer the effects of the cold. In regards to diving, this is a serious issue. You increase your risk of decompression sickness (DCS) enormously.
When diving in cold water, a dry suit is essential. Don't buy into the hype of semi-dry wet suits or thick 7mm wet suits. They work for a limited time and limited depth, but still reduce the body's core temperature to the point of legitimate concern. Also, when using a dry suit, using wet suit gloves also defeats the purpose. Dry gloves such as those sold by Si-Tec or Typhoon are ideal.
Even with a dry suit, there is littel to seperate your body from the cold waters without the proper undergarment. a 400 gram thinsulate such as sold by DUI is one of the best options you can get, but they are pricey. If you want to save some money, you can get something by Weezle, but they compress and can reduce the effectiveness at depth. I highly recommend thinsulate as it will not compress and gives you a steady and constant thermal protection.
Well, I hope this helps, if you have more questions please feel free to ask more.
