Is it permissible to travel on a boat during Shabbat, if one boarded the ship before Shabbat?
Clearly, one may not ride on a boat on Shabbat if the captain or crew members are Jews who do not observe Shabbat. Since the trip entails Shabbat desecration by Jews, it is forbidden to participate in the voyage. This is the ruling of Hacham Ovadia Yosef in his work Yehaveh Da'at (6:16) and in Yabia Omer (vol. 4, Orah Haim 43:6), as well as of Rabbi Shemuel Pinhasi (contemporary), in his work Ve'daber Davar (listen to audio recording for precise citation). Of course, as Rabbi Pinhasi notes, if the trip is necessary for the purposes of Piku'ah Nefesh saving a life then one may travel on the ship even if it is operated and maintained by Jews.
If the boat is operated and maintained by gentiles, as is more often the case, under what circumstances may one travel on the ship on Shabbat?
According to Halacha, if one wishes to embark on the trip for the purpose of performing a Misva, then he may board a ship at any point before Shabbat and remain on the ship on Shabbat. If, however, one travels for personal or recreational purposes, then Halacha forbids boarding a ship within a few days of Shabbat. According to some authorities, one may board a ship through Tuesday, whereas others allow boarding a ship through Wednesday. Rav Pinhasi writes that it is preferable, when possible, to follow the stringent position and not board a ship past Tuesday. If, however, one already made reservations or cannot find a suitable cruise before Wednesday, then he may follow the lenient view.
Hazal enacted this prohibition forbidding boarding a ship within a few days of Shabbat because doing so will likely interfere with one's "Oneg Shabbat" the enjoyment of Shabbat. It often takes several days to accustom oneself to sea travel and overcome seasickness, and so the Sages forbade boarding a ship too close to Shabbat in order to ensure that one will be comfortable and at ease on Shabbat.
Interestingly, Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahava (1:2), raises the possibility that this enactment of Hazal might not apply nowadays, when boats are very large and stable. Furthermore, most people who travel on cruises are accustomed to sea travel and thus do not become seasick. Nevertheless, the vast majority of Halachic authorities dispute this position and forbid boarding a ship after Tuesday (or Wednesday), unless one travels for the purpose of performing a Misva.
Thus, people who plan a cruise must ensure that the ship leaves no later than Wednesday, and preferably no later than Tuesday.
It is permissible to board a docked ship on Shabbat, provided that it will not be departing at all during Shabbat. However, if one travels on a ship on Shabbat and the ship makes a temporary stop at some point during Shabbat, he must ensure not to disembark from the ship, as it will be forbidden for him to reboard the ship again later before the ship departs.
Summary: It is forbidden to travel on Shabbat on a ship that is operated or maintained by Jews. If the ship is run by gentiles, one may ride on the ship on Shabbat provided that he boards no later than Wednesday, and preferably no later than Tuesday. If he travels for the purpose of a Misva, he may board anytime before Shabbat. It is permissible to board a docked ship on Shabbat, unless it will be departing at some later point during Shabbat.
