Below is information pulled directly from the "Marine Safety Information Bulletin" USCG MSIB 02-21 Ch-2 

The Centers for Disease Control and Prevention (CDC) issued an Order pdf icon[PDF – 11 pages] on January 29, 2021 requiring the wearing of masks by people on public transportation conveyances or on the premises of transportation hubs to prevent spread of the virus that causes COVID-19. This Order was effective as of 11:59 p.m. February 1, 2021 and was published in the Federal Registerexternal icon on February 3, 2021. CDC will be amending this Order as soon as practicable, to not require that people wear masks while outdoors on conveyances or while outdoors on the premises of transportation hubs.

On June 10, 2021, CDC announced that, until it can amend the January 29, 2021, Order, it will exercise its enforcement discretion regarding certain aspects of the Order to not require that people wear masks while outdoors on conveyances or while outdoors on the premises of transportation hubs. CDC requests that Federal partners and any cooperating state and local entities exercise similar enforcement discretion. This announcement does not affect any existing exemptions or exclusions of the Order. Subject to how other federal partners and state and local entities define “outdoors,” CDC understands “outdoors” to refer to any open-air area.

Conveyance operators must continue to require all people onboard to wear masks when boarding and disembarking, and for the duration of travel, unless they are located in outdoor areas of the conveyance (if such outdoor areas exist on the conveyance). Operators of transportation hubs must require all persons to wear a mask when entering or while located in the indoor premises of a transportation hub.

All passengers on public conveyances (e.g., airplanes, ships*, ferries, trains, subways, buses, taxis, ride-shares) traveling into, within, or out of the United States (including U.S. territories) as well as conveyance operators (e.g., crew, drivers, conductors, and other workers involved in the operation of conveyances), regardless of their vaccination status, are required to wear a mask over their nose and mouth. Unless otherwise required by the operator, or federal, State, tribal, territorial, or local government, people are not required to wear a mask when located in outdoor areas of a conveyance (if such outdoor areas exist on the conveyance).