Cruise ships have been doing a great job, preventing the outbreak of Covid infections of Staff and Guests, as well as having a majority of the cruises happening without a single case of Covid appearing. I can't do justice to the amount of information out there but I will provide a Link to the CDC website dealing with cruise ships and their individual status as well as a small info-bite about guidelines.
Info-bite:With the exception of cruise ships operating in Florida, all cruise ships operating in U.S. waters, or seeking to operate in U.S. waters, must comply with the requirements of the CDC Framework for Conditional Sailing Order (CSO) and Technical Instructions, even when outside of U.S. waters.
CDC - Cruise Ship Color Status | Quarantine