The Coast Guard Assistant Commandant for Prevention Policy has published Marine Safety Information Bulletin 02-21 Change 3 “COVID-19 Safety Requirements in the Maritime Transportation System” to provide notification that mariners on “Non-passenger- carrying commercial vessels operated by a team of mariners who all live on the vessel” are exempt from wearing a mask.
Source: USCG
Link: COVID-19 Safety Requirements in the Maritime Transportation System: Change-3