While US State and Federal Law applies on board cruise ships at port and within the territorial waters of the US, once the ship is 24 miles from shore they're in international waters and follow the laws of the country where the ship is registered/flagged. Cruise ships are rarely flagged in the US. Royal Caribbean, for instance, is flagged out of Liberia. It could very well be that the countries where these ships are flagged require proof of vaccination. In which case, the cruise lines are caught between the requirements they'll have to follow in international waters vs territorial waters & at port.
Source: https://people.howstuffworks.com/cruise-ship-law1.htm |
