The reason why cruise ships are not nuclear powered is because of public perception, regulatory hurdles and even logistical challenges.
Nuclear power does have some significant advantages such as zero emissions and extended operation although it has drawbacks that make it impractical for the cruise industry and cruise ships.
Dealing with nuclear reactors to power nuclear machines requires specialized people that are specially trained to work with nuclear to avoid any health issues or catastrophic events.
Most people would also be hesitant to go on a cruise ship that had a nuclear reactor, especially have the well publicized nuclear accidents such as Chernobyl.
There's also regulatory issues that cruise ships would face as nuclear powered ships have more complex regulations that they face and must comply with and even could be potentially banned from entering certain ports, which include ports with anti nuclear policies.
Special documentation as well as training for port personnel and emergency protocols are also required for any nuclear powered ships which also add to the complexity of operation.
And building and maintaining a nuclear reactor is more expensive than conventional diesel engines for diesel powered cruise ships and the space for the nuclear reactor and it's support systems that are needed can reduce passenger capacity.
And even with the advanced safety features, cruse ships that are nuclear powered would face the risk of collisions, grounding as well as other accidents which can potentially lead to radioactive material release.
There's also the issue with nuclear waste disposal which is a significant issue, and the cruise industry would have to have a plan to safely store and transport any spent nuclear fuel.