This applies to older adults, people with certain medical conditions and pregnant and recently pregnant people.
Before this, CDC had advised people who are not fully vaccinated to avoid cruise travel, worldwide, including river cruises.
Friday's CDC update came as the Delta variant sends COVID-19 cases soaring in parts of the US, with the daily average 145,913 on Aug. 20, a 37% increase over 14 days, according to The New York Times.
'The virus that causes COVID-19 spreads easily between people in close quarters aboard ships, and the chance of getting COVID-19 on cruise ships is high,' CDC said. 'Outbreaks of COVID-19 have been reported on cruise ships.'
The agency also advised that cruisers should wear a mask when in shared spaces and noted that while the US is not enforcing its mask order on cruise ships, individual lines may require travelers to wear masks indoors.
That is increasingly the case.