Even cruise lines did it voluntarily the CDC has ordered another No Sail order thru September 30th. Part of their reasoning crew still remaining on board. This is of no fault of the cruise lines when you have countries that refuse to take their own citizens in. 

Yet, we still have no mandates for the airlines. Many  that are not doing temper checks or practicing social distancing. They also use re circulated air. 

Sorry, but the CDC is very hypocritical when it comes to the cruise lines.