47 U.S.C. § 353a: Operators and watches on radiotelephone equipped ships

(a) Each cargo ship which in accordance with this part is equipped with a radiotelephone station shall, for safety purposes, carry at least one operator who may be the master, an officer, or a member of the crew. (b) Each cargo ship of the United States which in accordance with this part is equipped with a radiotelephone station shall, while being navigated in the open sea outside of a harbor or port, maintain continuous watch whenever the station is not being used for authorized traffic.