Going on a boat within 3 days of Shabbos
Yesterdays daf (Shabbos 19a) has the din that you are not allowed to go on a boat trip 3 days before Shabbos (if the trip will last over Shabbos). The Rishonim offer a whole host of possibilities what the reason for the issur is.
Rif, Rambam - it is because of oneg shabbos, in the first 3 days you will probably be seasick
Tosafos - a gezera that you aren't allowed to go on water
R' Chananel - it is talking about very shallow water (less then 10 tefachim) and is a problem of techum shabbos
Baal Hamaor - it is talking about where you will most probably have to do melacha for pikuach nefesh (boat trips were dangerous then) on shabbos
Ramban - it is talking about where the only people on the boat are Jews so the non-Jew is doing melacha for a Jew.
The Shulchan Aruch (סימן רמ"ח)seems to go with the Rif and the Rambam, however the din of techumim and pikuach nefesh is mentioned as well.
It comes out based on the above that nowadays it would be permitted to go on a cruise ship on Friday because none of the reasons (except maybe for Tosafos which is not accepted) apply.
However, this din is quoted l'halacha in other circumstances. If a person is having elective surgery (e.g. hernia, knee operation, etc.) where the surgery is not urgent, then they should not schedule it within 3 days of shabbos if possible for 2 reasons.
1. Oneg shabbos, the first few days after surgery are the most painful and will take away from oneg shabbos
2. Pikuach nefesh - the first few days after surgery can be dangerous and according to the Baal Hamaor you are not allowed to put yourself in a situation (within 3 days of Shabbos) where there is a good possibility that you will need to do melacha for pikuach nefesh.