Thermal wear will keep your body warm in the water. When swimming or snorkelling in colder waters, consider at least wearing a thermal top. Not only is the top of the body the spot where you lose your body heat first, it is also the spot that needs the most sun protection in the water. Thermal tops prevent your shoulders and back from getting burned. Thermal pants keep your body warmth from leaving your body rapidly in your groin area, another part of your body that you need to keep warm to prevent hypothermia.
The heat reflecting capabilities of thermal tops and thermal pants make them as good as a 2mm neoprene wetsuits but thermal tops are only 0.5mm thick. Thermal wear keeps you warm without affecting your manoeuvrability.
Thermal tops and thermal pants can also be worn under thinner wetsuits for extra warmth on the core of your body in winter, sparing you from having to buy another wetsuit for the colder months.