Traveling on a cruise ship brings with it a host of halachic concerns regarding shmiras Shabbos (observing the laws of Shabbos): If the captain and/or the crew are Jewish, we would be responsible for their inevitable chillul Shabbos (desecration of the Shabbos). Likewise, if half or more of the passengers are Jewish, any forbidden work done by the non-Jewish crew on Shabbos would be considered to be directly for our benefit. Finally, if the cruise company is owned by Jews, which is especially likely if the boat is Israeli, we would be abetting the owner in hiring non-Jews to do work on Shabbos. Therefore, being on board for Shabbos in any of those cases is prohibited. Assuming we have avoided all of those scenarios, departure would still have to take place three days prior to Shabbos (or Yom Tov), which is before Tuesday night; there are exceptions to this rule, and a rav should be consulted to determine the halachah in a given situation. Additionally, disembarking on Shabbos may be a violation of the laws of techumin (the distance one may walk on Shabbos), and we obviously may not carry our personal belongings off the ship until Shabbos is over. While on board the ship, care must be taken that we don’t trigger any of the electronic automatic doors on Shabbos, that a non-Jew uses the electronic keycard to open the doors to our cabins, and that food isn’t being heated up on our behalf.