Question

Which term describes a vessel that has turned on its side or has turned completely over?

Answer 1

The term is called capsizing or keeling over. It occurs when a vessel, such as a ship or a boat, has turned on its side or has turned completely over in water. There are numerous ways in preventing capsizing, such as the installation of various technologies.

Answer 2

Answer;
Capsized
The term that describes a vessel that has turned on its side or has turned completely over; is capsized.
Capsizing occurs when a vessel is turned on its side or it is upside down in the water. Righting is the act of reversing a capsized vessel. 
When a vessel has enough flotation to prevent sinking , it may recover on its own.