Do not set out to travel by boat on Thursday, Friday or Shabbos. Nor on Yom Tov, Yom Tov shayni, or three days before. The reason is that you may feel sea-sick and nullify the mitzva of Oneg Shabbos. (ShA1,2 – MB4,5)
A list of exceptions:
Going for a mitzva purpose. It exempts you from Oneg Shabbos. You must stipulate with the captain beforehand to dock for Shabbos. If he says no, you can still go.
Planning to arrive before Shabbos.
River travel does not cause motion sickness. If the river is known to be shallow so that the bottom of the boat is within 10 tefachim of the river bed it is still forbidden to embark before Shabbos because you will be traveling out of the techoom. (ShA1,2 – MB3)
In the above shallow river case, you can be lenient if you need to go, as long as you keep yourself on a deck which is above 10 tefachim. (MB14)
The beginning of the week is a time that you don’t yet need to take precautions for the upcoming Shabbos. Therefore you may embark even though the boat will be in shallow water and even if you will be needed to help sail the boat by doing melacha. (Rama1 – MB8)
In the case that you are allowed to be on the boat for Shabbos (e.g. mitzva purpose) You can even set sail on Shabbos day itself as long as you made the boat your dwelling place for Shabbos. That is you boarded the boat on the eve of Shabbos and stayed there for all of bayn hashmashos. You can exit at nightfall, go “home”, and on the morrow embark with all the other passengers. One condition: The boat must be floating above 10 tefachim from the sea/river bed. (ShA3 – MB20)
If you make the boat your home for Shabbos and you stay on the boat from the eve of Shabbos until it sets sail you are even allowed to exit the boat (if it docks) on Shabbos and walk up to 2,000 amos from the boat. (MB22)
You are not allowed to put yourself into a situation that will definitely require chillul Shabbos, even at the beginning of the week. (BeH “oo’fosayk”)