For the purposes of the International Rules of the Road, a non-self-propelled, semisubmersible drilling unit under tow is considered to be a __________.

Correct Answer: Option A — vessel

Rule 3 of the COLREGS defines a 'vessel' broadly to include every description of watercraft used or capable of being used as a means of transportation on water, including non-self-propelled semisubmersible drilling units.