The Halacha prohibits boarding a boat within three days of Shabbat for a journey through Shabbat. This restriction only applies to travel not for the purpose of a misva. For example, one is allowed to embark even on Friday afternoon to make Aliya to Eres Yisrael. Business travel, on the other hand, is not considered a misva purpose in this context.

With regard to defining “within three days,” Hacham Ovadia follows the opinion that Shabbat is included in that time span. Therefore, it is permitted to embark from Wednesday.

The Rishonim (Early Commentaries) presented various reasons for the rationale of this prohibition. The Shulhan Aruch (248) adopts the position of the Rif and the Rambam on this issue. They understand that sailing within three days of Shabbat is liable to diminish one’s Oneg Shabbat (Enjoyment of Shabbat), because of the unpleasant salty air and the disorientation and seasickness caused by travel on water. If one embarks on Wednesday or earlier, he will have sufficient time to become acclimated to the conditions, in order to enjoy Shabbat properly.

According to this reasoning, the Shulhan Aruch permits sailing on fresh water, such as rivers, because there is no foul-smelling salty air. The Shulhan Esh deduces from this leniency that travel is only prohibited if both negative factors are present: Foul air and turbulence. Even though there is disorientation from the turbulence in fresh water, the Shulhan Aruch permitted travel because there is no salt air.

SUMMARY:
It is prohibited from Thursday to embark on a boat that will travel through Shabbat. Such travel is permitted if the travel is for a misva or on fresh water.