A vaccine is not 100% and also does not mean you can't spread COVID19 as well.
Lets say they do 50% of cabins on the fantasy. Thats 625 cabins and likely 1875 total guests. A 90% effective rate leaves 187 passengers unprotected in addition the other 1700 passengers could be carriers
No, it doesn't work like that. If you have been vaccinated, it prevents the virus from "connecting" to your cells and injecting the required mRNA sequence to manufacture more viruses. In short, if you have been vaccinated using the current vaccines that are in consideration by the FDA (and it 'takes'), you can not be a carrier. If everyone on the ship has been vaccinated, the maximum number of people that could be carriers would only be 187.
The only way those 187 would be all be carriers is if 5 days before the cruise they got together in a mass infection orgy and coughed all over each other for a few hours.
FAR, FAR, FAR more likely would be 1-2 of those people MIGHT be carriers. In an absolute worst case scenario, if a number of them got together in a
DVC party or something on the first day, they might spread it to another 4 people. By the end of the cruise, those 4 might have been contagious, and if they have a going away party, might be able to infect another 2-4 people. So by the time everyone gets home you might have 6-8 people IF YOU WERE VERY, VERY UNLUCKY. Far more likely might be 1-2 people getting infected
The exposure to the Bahamas would still be the 1-2 + maybe 1-2 more. If you add a rapid test on top of this prior to boarding, those chances drop significantly more.