Autonomous underwater vehicle.

From the Wikipedia page:

An autonomous underwater vehicle (AUV) is a robot which travels underwater without requiring input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes non-autonomous remotely operated underwater vehicles (ROVs) – controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. In military applications AUVs more often referred to simply as unmanned undersea vehicles (UUVs).

Note that:

• Questions about autonomous Unmanned aerial vehicles should use the uav tag.
• Questions about autonomous Unmanned ground vehicles should use the ugv tag.
• The drone tag should only be used on questions about autonomous drones which don't fit into the auv, uav or ugv tags.
• Questions about Remotely operated underwater vehicles are probably off topic on Robotics and are more likely to be suitable over on Electrical Engineering Stack Exchange.

If the tags uuv & rov start to be used, then they should probably be considered synonyms of auv.