Thanks to the latest developments in exposure protection, divers can stay warm without a bulky wetsuit. The makers of neoprene foam, who supply the wetsuit makers, had only recently discovered how to slice the material as thin as 0.5 mm, then bond it to nylon cloth. When bonded to nylon, the material is about the thickness of a dime.
Neoprene is waterproof.
Water can't penetrate the fabric to your skin. When this thin, neoprene is very stretchy, so it hugs your body without the wrinkles and bags that would pump water in and out whenever you move. Trilaminates are more waterproof than fleece but less stretchy. Only super-thin neoprene both stretches and seals well. Those qualities together practically eliminate water flow over your skin.