Gemara

(Beraisa): One may not embark on a ship less than three days before Shabbos.

This refers to a voyage of Reshus, but it is permitted if he goes for a Mitzvah.

Rebbi says, he must stipulate with the captain that they will not travel on Shabbos. [Later,] we are not concerned if they actually do travel on Shabbos;

R. Shimon ben Gamliel says, he need not stipulate.

One may go from Tzur to Tzidon even on Erev Shabbos. (The voyage takes only one day.)

Rishonim

Rif and Rosh (Shabbos 7b and 1:38): The Isur to embark less than three days before Shabbos is due to Bitul Oneg Shabbos. For three days, this is a change in his Veses (normal state) from the movement of the ship - "Yachogu v'Yanu'u ka'Shikor", and he cannot fulfill Oneg Shabbos. We permit due to a Mitzvah, for he is exempt from Oneg, since one engaged in a Mitzvah is exempt from [another] Mitzvah; people on the way to a Mitzvah are exempt from Sukah (Sukah 25a).

Rambam (Hilchos Shabbos 24:6): One may embark on a voyage on Erev Shabbos in the Yam ha'Gadol for a Mitzvah. He stipulates with [the captain] to cease [on Shabbos], and he does [need] not cease.

Rambam (30:13): For a Mitzvah one may embark even on Erev Shabbos. From Tzur to Tzidon or similar trips it is permitted even for Reshus even on Erev Shabbos. Where the custom is not to embark on Shabbos at all, we do not embark.

Magid Mishneh: Nowadays the custom is to embark even to the Yam ha'Gadol on Erev Shabbos and Shabbos. The Ramban sought to defend this, and said that a Ba'al Nefesh should be stringent for himself. Also the Rashba said that his words are wrong.

Rosh (ibid.): The Rashbam says that the Beraisa is like Beis Shamai, who forbid Melachah that occurs by itself on Shabbos, e.g. the ship leaving the Techum. Since we hold like Beis Hillel, one may embark on Erev Shabbos. It is forbidden on Shabbos. The Tosefta (Korban Nesan'el - Yerushalmi) supports this. It says 'one may not embark on a ship three days before Shabbos Shabbos. Beis Shamai forbid on Wednesday. Beis Hillel permit. The Rashbam wants to say that Beis Hillel permit even on Erev Shabbos. If they permit only on Wednesday, this is like the first Tana!

Rebuttal (Rosh): This is difficult, to say that Rebbi and R. Shimon ben Gamliel discuss Beis Shamai's opinion! He testified that his household were stringent for themselves about laundry, but he does not come to rule like this for others! Also, since Techumim is mid'Rabanan, Beis Shamai should not be stringent. They do not forbid beginning before Shabbos a Melachah mid'Rabanan that will occur by itself. All permit loading beams on olives or grapes for this reason (19a). The Yerushalmi says that Beis Hillel permit, i.e. on Wednesday. This is unlike the first Tana, who held that Beis Shamai do not disagree about this. Even if we would say that Beis Hillel permit on Erev Shabbos, we do not abandon the Beraisa in the Bavli, in which latter Tana'im forbid. Presumably they do not argue with Beis Hillel. Rather, they say that Beis Hillel and Beis Shamai did not argue about this.

Poskim

Shulchan Aruch (OC 248:1): One may embark on a boat even on Erev Shabbos, if he goes for a Mitzvah.

Mishnah Berurah (1): One may embark for a Mitzvah even though he surely will not have Oneg Shabbos, and even if the water is less than 10 Tefachim.

Magen Avraham (1): One may not embark on Shabbos even for a Mitzvah, for he cannot stipulate that he not go on Shabbos.

Mishnah Berurah (2): Some permit even on Shabbos for a Mitzvah. See Sha'arei Teshuvah at the end of this Siman, and Chayei Adam 4:1.

Shulchan Aruch (ibid.): He stipulates with him to cease [on Shabbos], and if he does not cease, there is no problem.

Beis Yosef (DH u'Mah she'Chasav Rabeinu): The Tur rules like R. Shimon ben Gamliel. The Rambam and R. Yerucham rule like the first Tana.Mishnah Berurah (3): Most Poskim say that the stipulation is only l'Chatchilah. It is not Me'akev if the Nochri refuses to stipulate.

Taz (1): One must stipulate not only on Erev Shabbos. Rather, even one who embarks within three days must stipulate. The Tur says so according to the first Tana. Why is this different than giving hides to a tanner or letters to deliver, which Beis Hillel permit just before dark, even though he will [tan or] deliver on Shabbos? Here the Yisrael sits in the boat, so Chachamim obligated him to do as much as he can to distance himself [from transgression of Shabbos]. There is no distinction between Erev Shabbos and before this.

Kaf ha'Chayim (4): If the Nochri refuses to stipulate, one may be lenient if he might lose the Mitzvah if he waits for another boat. This is if he will not need to do an Isur on Shabbos. If he will need to do an Isur, he must stipulate.

Shulchan Aruch (ibid.): For Reshus one may not embark less than three days before Shabbos.

Rema: Before three days it is permitted even in a boat drawn by animals, even if the water is less than 10 Tefachim deep, even where the Yisrael will need to do Melachah on Shabbos to move the boat.

Taz (2): We are not concerned for Techumim because he began when it was permitted.

Taz (3): The three days after Shabbos pertains to the previous Shabbos, so he need not refrain due to the coming Shabbos. If he will need to do Melachah on Shabbos for Piku'ach Nefesh, this is not Chilul Shabbos.

Magen Avraham (2): Wednesday is forbidden. Also Yom Tov Sheni is forbidden.

Gra (DH Aval): One may embark [on a ship drawn by animals] even within three days, like [the Rema] says in Sa'if 2. The Mechaber mentions three days due to the other matters. Alternatively, he forbids three days because they are considered before Shabbos, like the Ran and Ba'al ha'Ma'or. However, the Rosh brings a Tosefta that connotes that Wednesday is permitted, and the Rashba brings a Yerushalmi that says so. Hagahos Ashri and Tosfos (Eruvin 43a DH Halachah) agree.

Gra (DH v'Afiliu Ein): This is unlike the Rif, who forbids even on Sunday. This is because the Gemara asked whether Techumim applies above 10 Tefachim. According to the Rif, the Gemara should have proved that it does not (because we permit embarking more than three days before Shabbos when the ship is above 10).

Mishnah Berurah (6): This is unlike a wagon. One may not sit in a wagon even if he entered three days before Shabbos. This is permitted regarding a boat drawn by animals (Rema, Sa'if 2).

Mishnah Berurah (8): Three days before Shabbos one must be careful not to come to Chilul Shabbos. Embarking within this time is like stipulating to desecrate Shabbos.

Shulchan Aruch (ibid.): If it is a small trip, e.g. from Tzur to Tzidon, which are only one day's journey apart, one may embark on Erev Shabbos in the morning, for he can reach there before Shabbos. Where the custom is not to embark on Shabbos at all, even a small trip, we do not embark.

Beis Yosef (DH u'Mah she'Chasav v'Derech): Rashi says that from Tzur to Tzidon is permitted on Erev Shabbos because they are only one day's journey apart. It seems that even though sometimes the trip takes more than one day, if it takes only one day when there is a good wind, it is permitted. Semag and Sefer ha'Terumah say so. This is only if he leaves early enough in the day that he can reach there before Shabbos with a good wind.

Bi'ur Halachah (DH she'Ein)): Similarly, if there is a two-day journey in between, one may embark on Thursday.

Rema (2): In the three days before Shabbos, one may not enter any boat in which the Yisrael will need to do Melachah on Shabbos, even in rivers that flow above 10 Tefachim.

Mishnah Berurah (16): This refers even to a Melachah mid'Rabanan.

Kaf ha'Chayim (21): It is permitted on Erev Shabbos if the boat goes for the sake of Nochrim.

Kaf ha'Chayim (24): It is permitted even if the entire crew is Yisre'elim, as long as they do Melachah only due to danger. One should not be a sailor if he will need to be Mechalel Shabbos, even if he leaves three days before Shabbos. He can find another job! The Ari Zal says that even though 'the Torah exempts Ones', there is a punishment and he needs a Kaparah.

Rema: There is no Isur for animals to pull the boat from the river bank. This is unlike going in a wagon (that animals pull), which is forbidden.

Taz (4): We forbid a wagon lest one get out and walk. This does not apply to a boat.

Magen Avraham (9): We decree about a wagon, lest he break a branch [to hit the animal, so it will go]. In the boat, the animals are far from him. Therefore, this is like Tzidei Tzedadim (i.e. something on the side of an animal - 305:18. One may use it, even though one may not use the side of an animal.)

Gra (DH Aval): This is even from Sunday, like the Rif. The Beis Yosef rejected Maharik. The Rema added "to leave", for the Isur is only to leave within three days. He says 'likewise', for three days applies also to what the Shulchan Aruch added.