Q. Can someone enter a boat that is tied up to a dock on Shabbat?
A. Shulchan Aruch (O.H. 339: 7) rules that one may enter a boat on Shabbos if the boat is sitting on the ground of the sea and not floating at all; and if it is tied up, in the way that boats are to the dock, even if it floats on the water one may enter. Mishna Berura (ibid. 35-37) explains that if the boat floats, one should not enter since it appears as if he is swimming. Being tied up, he adds, is properly on both extremes of the rope. Shemiras Shabbos K’ (16: 38) also permits entering a small boat or sirah, even when floating on the water, if it is tied up properly. See also Shulchan Aruch (O.H 613: 6) regarding traveling on such a boat for the purpose of a mitzva. See also next questions in regard to carrying items to and from the boat on Shabbos.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a