I hadn't worn a wetsuit before but was offered use of one for a dip in the sea. I have often wondered about why a swimming pool at 18 degrees feels much colder than the air at 18 degrees. I think that this is because the particles are closer in the water so it can conduct heat better. The wetsuit seems to work by trapping a layer of water and heating only that. As long as that remains next to the skin and is not replaced by cold water, thermal equilibrium is reached between the body and that layer of water so the body doesn't cool down further. The outer rubber layer means that the trapped layer of water loses heat slowly to the surrounding sea. It is well explained in detail at https://www.explainthatstuff.com/howwetsuitswork.html
