One of the first simanim in shulcan uruch hilchos shabbos is about יוצא לים בספינה.
The mishna brerura in his preface discusses 4 issues:
1. Oineg shabbos. The shu'e himself says this only applies in types of water where it takes time to get used too, so aussie is right with this one.
2. Techumin, while essentially there is no issur techimn while travelling wholly above 10 tefuchim, it seems there might be a problem.
3. A chashesh of chillul shabbos as a result of a later emergency.
4. A geizara derbaban, aletz another method of travel which banned. (When travelling >3days before, there is heker and rabanan weren't goizer)
I didn't read into the enitre siman, but I'd suggest you look there.
(P.s. As SF mention, the Shu"a alows departing even <3 days when it is letzoerch mitzva)