One should not embark on a boat on the sea
less than three days before the start of Shabbat, 1
that is, from Wednesday onwards, it is forbidden. 2
However, if one is journeying for the sake of a mitzvah 3
it is permitted even on Friday.1) There are varying opinions among the commentators as to the basis of this prohibition, however, the one accepted by the Shulchan Aruch is that travelling on the ocean upsets one's system (sea-sickness, etc.) in a way that would detract from one's enjoyment of Shabbat (Oneg Shabbat), and it can take up to three days to recover. Since this reasoning doesn't apply to travelling on a river, these restrictions wouldn't apply.
(2) This is the ruling of the Magen Avraham (248:2) and the Shulchan Aruch HaRav (248:1). The Vilna Gaon, however, maintains that Shabbat is included in these three days, and so the prohibition begins on Thursday.
3) Some authorities rule that if one is going on a business trip or even to visit a friend, then it is the equivalent (within the context of this particular prohibition) of travelling to perform a mitzvah, and one may therefore embark on Friday. According to these authorities, only leaving for a vacation is prohibited less than 3 days before Shabbat.
אין מפליגין בספינה על הים
פחות משלשה ימים קודם שבת
דהיינו מיום ד' ואילך אסור
אבל אם הולך לדבר מצוה
מותר אפילו בערב שבת On rivers one is permitted to embark on a boat,
under all circumstances, even on Friday, 1
as long as the Jew is not required to do there (forbidden) Shabbat work.
Even if animals are drawing along the boat, it is permitted. 21) Travelling on a river, according to the Sages, does not mess up one's system in the way that a sea-journey would; therefore, embarking on a riverboat on Friday would not detract from one's Shabbat enjoyment and is permitted.
(2) It is Rabbinically forbidden to ride an animal or even in an animal-drawn carriage on Shabbat, because the Sages feared that one might inadvertently snap off a tree branch with which to hit the animal. This prohibition doesn't apply to boats which are pulled down the river by animals walking on the riverbank, because the animals are a great distance from the boat itself.
על הנהרות מותר להפליג בספינה
בכל ענין אפילו בערב שבת
ובלבד שלא יצטרך הישראל לעשות שם מלאכה בשבת
ואפילו אם בהמות מוליכות את הספינה מותר Under what manner is it permitted to embark on a ship on Friday ?
If one went on to it on Friday and remained there until night,
even though one returned to one's home and slept in one's home,
one is permitted to go on board on Shabbat day 1
as long as the ship is not traveling for the sake of Jews alone.
However, since one has been at one's home on Shabbat
one acquires (again) that as one's residence 2
so if the ship traveled beyond the Shabbat boundary,
and arrived to dry land (a port) one only has four cubits, 3
and more than this one is not allowed to go.1) One is not allowed to travel more than 2000 cubits on Shabbat, from the place one establishes one's ''residence'' at the onset of Shabbat; this law is based on the verse (Exodus 16:29): ''A man shall not leave his place on the seventh day (i.e. on Shabbat)''. The area within the 2000 cubits is called one's ''Shabbat Boundary''. One of the reasons that it might be forbidden to board a boat on Shabbat, is that it looks like you are intending to travel beyond 2000 cubits on Shabbat. However, if one boards the boat on Friday, and remains there until it gets dark (some require even making Kiddush and eating a little on the boat), then the boat is seen as one's ''home'' for Shabbat, and one may re-enter it on Shabbat itself (Aruch Hashulchan 248:13).
2) Remaining on the ship until nightfall on Friday night only serves to allow him to re-enter the ship on Shabbat day. However, because he left the boat on Friday night and spent the night in his house, his house is considered to be his true Shabbat residence; therefore, if the ship travels more than 2000 cubits from his house on Shabbos, he may not leave the confines of the ship. (Mishna Berurah 248:22).
3) One may walk through the entire ship, but not disembark. See also Chapter 95.
היכא דמותר להפליג בספינה בערב שבת
אם נכנס בה בערב שבת וישב שם עד חשיכה
אף על פי שחזר לביתו ולן בביתו
מותר ליכנס בה אחר כך ביום השבת
ובלבד שלא תלך הספינה בשביל ישראלים לחוד
אך מאחר שהיה בביתו
בשבת קנה שביתה בביתו
ולכן אם הלכה הספינה יותר מתחום שבת
ובאה בשבת ליבשה אין לו שם אלא ארבע אמות
ויותר מזה אסור לו ללכת שמה Going on a ship in order to pray with a minyan or for another mitzvah,
provided the ship is traveling for others (then this) can be allowed.
In any event the Jew should go onto the ship on Friday,
while it is still day and stay there until it is dark,
afterwards one may return to one's home
and come back on Shabbat to the ship.
However if the ship is travelling for the sake of Jews alone, 1
this should not be allowed.1) See the N'tiv Chayim on the Shulchan Aruch, Chapter 248, for more details.
ללכת בספינה כדי להתפלל בעשרה או למצוה אחרת
אם הספינה הולכת גם בשביל אחרים יש להתיר
מכל מקום יש להישראל ליכנס בתוך הספינה בערב שבת
בעוד יום ולישב שמה עד שתחשך
ואחר כך יכול לחזור לביתו
וחוזר בשבת לתוך הספינה
אבל שתלך הספינה בשביל הישראל לבד
אין להתיר