Indoor congregant settings are where the virus spreads most easily. Cruise ships require very high rates of vaccination in order to sail. They’re likely safer than spending time indoors at restaurants, bars and gyms or attending indoor concerts and sporting events. But they’re going to have Covid-19 cases because breakthrough infections are real, and they’re looking for cases.
It’s not the health risk you should be worried about, it’s whether you’ll be able to visit the ports on your itinerary, whether you’ll be quarantined in your room, and whether you’ll be allowed to disembark and return home as-planned.