You may not board a sea-going vessel less than three days before Shabbos;¹ (enjoyment of) Shabbos. It takes three days to adjust and recover from sailing in the salty waters of the sea. Such voyages often upset one’s system. [Rif etc. Shulchan Aruch 248: 2] that is from Wednesday on, it is forbidden. But if you are going to perform a mitzvah,² 248: 4 and Mishnah Berurah. you may embark even on Friday.

footnotes: ¹ So as not to disturb your oneg ² Some authorities hold that it is permitted even for a business trip—anything other than vacationing. Other authorities hold that only for a mitzvah is it permitted. See Rema

[If the boat is to sail] on rivers³. you may embark for any purpose, even on Friday, provided that a Jew need not perform melachah⁴ on Shabbos; that means the water must be ten handbreadths (tefachim) deep from the bottom of the boat to the floor of the river [See Shulchan Aruch 248: 2]. on Shabbos. And even if animals pull the boat, it is permitted.

footnotes: ³ Sailing on rivers usually does not disturb one’s oneg Shabbos ⁴ And you need not travel out of the techum

(In cases) where it is permitted to board ship on Friday, and you boarded the ship on Friday and remained there until dark, even though you went home and remained overnight at home, you are permitted back on board ship on Shabbos,⁵]. provided the ship does not make the trip exclusively for Jewish passengers. However, since you were in your house on Shabbos, you established shevisah [residence] in your house,⁶ refers to your residence on that Shabbos. Your place of shevisah is the focal point of your boundaries [techum]. The city in which you reside is your place of shevisah. Your techum is described in chapter 95. If you leave your place of shevisah on Shabbos, you must remain within four amohs of your point of exit. therefore, if the boat sails more than the techum Shabbos and reached land on Shabbos, you are permitted to walk only four amohs, but it is forbidden to walk more than that from the landing.⁷ 248: 11]. See chapter 95.

footnotes: ⁵ It will not appear as though you are setting out to travel on Shabbos, but rather as though you are going into your house [Magein Avraham ⁶ Shevisah ⁷ But on the boat itself there are no restrictions, as long as you boarded before Shabbos. However, if you boarded on Shabbos you may only walk four cubits even on the boat [Magein Avraham

It is permitted to board a ship on Shabbos in order to pray with a minyan of ten, or for the sake of another mitzvah, provided the ship is making the trip for others as well. However, you should try to board the ship on Friday while it is yet day and remain there till nightfall, after which you may return to your home, and come back to the boat on Shabbos. But if the vessel makes the trip only for the sake of a Jew, it is not permitted.