An underwater craft is a type of vessel that is designed to operate beneath the surface of the water. The term 'undersea craft' can refer to any type of vessel, from submarines to remotely operated vehicles (ROVs). Submarines are the most common type of undersea craft, and they are typically used for military purposes such as surveillance and reconnaissance. ROVs are typically used for scientific research and exploration, and they are often equipped with cameras and other sensors to collect data from the ocean floor. The term 'SUB' is an abbreviation for submarine.