I don’t think most people who are vaccinated with one of the mRNA vaccines in widest use in the United States have too much to fear from Covid-19, and shouldn’t be overly concerned about catching the virus on a cruise ship. However I still don’t think it’s a good idea to cruise now, even if you’re vaccinated.
That’s because other people might catch the virus, and you might be forced to quarantine. Or worse yet, your ship might not be able to dock. I’d want greater certainty that a cruise is going to end according to its published itinerary before recommending a return to sailing.