74:1 A person should not embark on a boat in order to go to sea less than three days before Shabbos, i.e., from Wednesday on. (Mishnah Brurah 248:4 cites an opinion that the three days include Shabbos itself, so one may board on Wednesday but not Thursday.) However, if one is going on the journey for the sake of a mitzvah, he may even embark on Friday.
74:2 One is permitted to embark on a boat on a river even on Friday for any reason so long as a Jew will not be required to perform forbidden acts of labor on Shabbos. One may board the boat even if it is drawn by animals. (If it is known that there is less than 10 handbreadths – about 30 inches – between the bottom of the boat and the river bottom, then one may not travel outside the Shabbos boundary – OC and Rema 248:2.)