An immersion suit is also called a survival suit. Marine Norway recommend you to wear an immersion suit at all times. It is a type of suit that actually works by protecting the wearer from hypothermia due to immersion in cold water by locking air and radiant body heat in the suit. Especially in large open water bodies for example oceans and seas. These occupations include sailing, pilots, fishing vessels, and gas and oil rigs.
For more information, see this page: http://www.immersionsuits.info.