A ship is one of the worst places to be when a pandemic is spreading on board. However, its one of the best places to be if you're avoiding a pandemic and everyone is healthy on board. The hard part about waiting it out is getting food to keep the people on board alive. Most ships have their own water desalination plants so water isn't really a concern. Another concern is fuel since you probably don't want to dock or anchor a ship full of food and supplies right off the coast of an infected city.