Wets Suits are deisgned to keep you warm in cold water. Available in different styles and thicknesses, you can normally match your diving environment to the suits type. A 3mm shorty may be all thats required in the tropics, where as a 5mm full length may be better of for the Red Sea in winter. A suit that stops water flushing through, or semi dry as its known, can keep you warm enough even in the UK.
