29 C.F.R. § 783.47
Off-duty periods include not only such periods as shore leave but also generally those hours spent by a seaman on the vessel outside his watch or normal or regular working hours and his standby periods during which hours he is not required to perform and does not perform work of any kind but is free to utilize his time for his own purpose. The fact that during such off-duty periods the employee is subject to call in case of emergency situations affecting the safety and welfare of the vessel upon which he is employed, or of its passengers, crew, or cargo or for participation in life boat or fire drills will not render such off-duty periods, excluded by employment agreement applicable to the employee, "hours worked". Responding to such calls, however, as well as the performance of work in response thereto constitute compensable work time. For further and more detailed discussion on what generally are regarded as "hours worked" under the Act, see part 785 of this chapter.
29 C.F.R. § 783.47