The passengers and crew could be all vaccinated but it sounds like they're doing ports of call, which basically exposes them to unvaccinated persons.
Last year, the longest hospitalized person with Covid in San Francisco was finally released. In the story, they detailed that he and his wife had traveled to South Africa, UK, over three months, then back to the US. The husband was infected. The wife didn't and tested negative.
What “didn’t” the wife do?