An underwater vessel is a type of watercraft that is designed to operate beneath the surface of the water. The most common type of underwater vessel is the SUBMARINE, which is a large, heavily-armored vessel that is capable of submerging for extended periods of time. A SUB, which is a shortened form of SUBMARINE, is also a type of underwater vessel. Uboat is another term for SUBMARINE, and is used primarily in reference to German submarines during World War II.