Tort liability for joint operation of a vessel is determined proximate to the time of the accident, not through the duration of the voyage. The boat owner therefore had no duty to supervise the skilled passenger under a duty of joint operation when the latter took the helm, and as there was no duty of care, no question of comparative negligence arises.
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/08/10/16-15942.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.