The neoprene of the wetsuit contains small bubbles of nitrogen gas. Nitrogen gas has a very low thermal conductivity. This prevents the heat inside the wetsuit from escaping to the water outside of the wetsuit. If you were in the water without a wetsuit on, the heat from your body would escape to the water because the water has a very high thermal conductivity.
The wetsuit does not prevent all of the water from getting inside the wetsuit. There is a thin layer of water that is allowed to get between the suit and the skin of the diver. The skin of the diver will warm the water. The water and the wetsuit will then serve as insulation against the cold water.
The warmth you get from the wetsuit will also be determined by the thickness of the neoprene. The suit will keep you warmer if the neoprene is thicker. The thickness is measured in millimeters. The largest size is 6 millimeters. Some manufacturers will use different thickness for different parts of the suit.
Another consideration that you might have, if you are concerned about staying warm, is how well your wetsuit fits on your body. If the wetsuit is too loose of a fit on your body, then the thin layer of water between your skin and the wetsuit might escape, along with some body heat. If the suit is not a close fit then cold water might rush into the suit, which prevents the water already inside the suit from warming up. If you use flexible seals around the openings near the feet, hands and neck, this might help keep the cold water out and the warm water in. You should choose a wetsuit that fits your body as tightly as possible, but it should not be so tight that it prevents you from having your full range of movement. You do not have to worry about the wetsuit shrinking when you get into the water, because they stay close to their original size.
There are neoprene wetsuits that have nylon fabric on the inside and a smooth surface on the outside. This is known as smooth skin neoprene. This type of wetsuit will keep you warmer because the smooth surface will absorb less water.
There is another type of neoprene wetsuit that has nylon fabric on both sides. This is known as nylon 2. The nylon 2 wetsuit is more durable than the smooth skin neoprene wetsuit. The nylon fabric will strengthen the wetsuit and make it easier to slip on and off. Neoprene wetsuits can be made using either one of two processes: chemical blown or gas blown. A chemical blown wetsuit feels softer and nicer. A gas blown wetsuit is more durable but it is also more expensive.
In the last several years, wetsuits containing merino wool and titanium thread have become popular. These suits are meant to help retain more body heat. These wetsuits are also more expensive than the neoprene wetsuits. There also are wetsuits that have no coatings and these are called skins. They fit very well and are very comfortable. They are hard to slip on and off, because they are very fragile.
