Wetsuits are made from a synthetic type of rubber called neoprene. As the name suggests, wetsuits don’t actually prevent you from getting wet.
But instead a wetsuit is designed to keep us warm using a thin layer of water. This thin layer of water enters the wetsuit and lies between the wetsuit and your skin.
Your wetsuit should be tight enough to fit snugly. But it shouldn't be too tight to cut off your circulation.
The snug fit should be so there's only a minimal layer of water that fills the 'space' between your wetsuit and your skin. This layer of water will be so thin that it is easily warmed up by your body heat.