Underwater craft is a term used to describe any type of vessel or vehicle designed to operate in an aquatic environment. This includes submarines, boats, ships, and other watercraft. Submarines are the most common type of underwater craft, and they are typically used for military purposes such as surveillance, reconnaissance, and combat operations. Boats and ships are also used for recreational activities such as fishing, sailing, and cruising. Other types of underwater craft include remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), which are used for research and exploration.