Translation:Shulchan Aruch/Orach Chaim/248
1. It is permitted to commence a sea voyage, even on a Friday, if he is traveling for the purpose of a Mitzvah. He must agree [with the captain] to stop on Shabbat; if he subsequently [breaks this agreement] and does not stop it does not matter. For personal reasons (i.e. not for a Mitzvah) one may not commence the journey within three days of Shabbat. RAMA: However, more than three days before [Shabbat] it is permitted, even in ships that will be dragged by animals and even if the depth of the water is less than ten hand breaths [Mahari"k]; even if a Jew will then be required to do a Melacha on Shabbat in order to steer the boat. [Rivash] If it a short journey, e.g. from Tyre to Sidon which is just one day's journey, one may commence travel on a Friday morning, since it is possible to arrive at the destination before Shabbat. In places where it is customary not to commence journeys on Friday at all, even short journeys, it is forbidden to do so.
2.The reason it is forbidden to commence a journey within three days of Shabbat is because of the enjoyment of Shabbat, since for the first three days travelers suffer discomfort and confusion. This only applies to sea voyages; travel in rivers do not cause any pain and it is therefore permitted to commence such a journey even on a Friday. This is only if we are not aware that the depth of the waters is less ten hand breaths; in places where we know that from the bottom of the boat to the bottom of the river the distance is less than ten hand breaths it is forbidden to travel outside of the Techum because of the prohibition of Techum. Likewise in a ship where a Jew will have to perform Melacha on Shabbat one should not enter three days before Shabbat, even if they are flowing rivers and the ship is above ten hand breaths. There is no prohibition, however, if animals have to drag the boat from the bank of the river and it is not the same as traveling in a wagon, which is forbidden.
3. 3. This - that it is permitted to commence a sea voyage on Erev Shabbat if he established it as his place for Shabbos on Friday evening, (i.e., he was on the ship from the beginning of bein hashamashos until nightfall), even if he travels on Shabbat itself - is permitted. This is so only if he does not leave the ship from the time he established it as his place (for Shabbat). RAMA: There are those who say that even if he leaves the ship it is permitted. And after he established the ship as his place from Erev Shabbat, it is permitted to enter into the ship on Shabbat and commence his voyage. And there are those who say that one makes Kiddush on the ship and then afterwards they may return to their houses and sleep there. The next day they return to the ship and begin to travel. This is the custom in a few places and we do not object (R"an; Rivash). See Siman 339.
4. If one goes out on a caravan in the desert on Shabbat and everyone knows they will be required to violate Shabbat because of the danger - they cannot tarry in the desert on Shabbat by themselves - for three days prior to Shabbat it is forbidden to go (on this caravan). On Sunday, Monday and Tuesday it is permitted to embark on this journey. If afterwards danger should happen to him and he is required to violate Shabbat because of preservation of the soul it is permitted and it is not considered a violation. One who goes up to the Land of Israel to immigrate may go via caravan even on Erev Shabbat. How so? Because it is for a mitzvah. He is allowed to leave and stop with them for Shabbat. And if after he was in the desert they do not want to rest with him, he is permitted to go with them outside the Techum because of the preservation of the soul, פיקוח נפש. If they come to one city on Shabbat he can walk throughout the whole city. And even if they rest outside of the city and they desire to enter the city, it is permitted. Why? For a mitzvah. He is able to walk 2000 amot in all directions. RAMA: There are those who say that every place a person goes for commerce, or to see a friend, it is considered the same as traveling for a matter of a mitzvah, but it is not considered for personal concerns except when he goes out for a stroll. Therefore it is the custom in a few places to be lenient regarding travelling by ship and going out on a caravan within three days because everything is considered like traveling for the sake of a mitzvah, and we do not object to this since there is one upon whom they can rely.