Not exactly true. See
https://www.cdc.gov/regulations/index.html "As the nation’s public health protection agency, CDC has certain authorities to implement regulations related to protecting America from health and safety threats, both foreign and within the United States, and increasing public health security."
In the case of cruise ships that use US ports or sail through US waters, among other things there are regulations that apply, guidelines that are issued, and right now the No Sail Order that is in place [see
https://www.cdc.gov/quarantine/pdf/No-Sail-Order-Cruise-Ships_Extension_4-9-20-encrypted.pdf ].
While the CDC doesn't create the laws, "CDC and other agencies implement public health laws passed by Congress through Federal Regulations. After a Congressional bill becomes law, federal agencies may be responsible for putting the law into action through the development of regulations—also known as “rules.” Federal regulations give the public details or specific requirements of how the law will be applied. The process of creating regulations or rules is called rulemaking." (quote from the first link above)