Can you go on a 7 day cruise that stops every day but leaves on Sunday?
Yes you may. The reason we aren’t allowed to go on a boat that is leaving less than three days before shabbos is because it takes three days to get used to the rough sea conditions, the sea’s waves and the salty smell, and it is ruining the person oneg shabbos. However, in this case since you started the trip beforehand, you are used to the conditions and it isn’t a problem. Besides there are poskim that say that nowadays that our boats are smooth and pleasant, it is not going to ruin the person’s oneg Shabbos even if he starts the trip later in the week.
O”CH 248-2, M:B Introduction to Siman 248, Hilchos Shabbos B’shabbos vol. 1 chap.2 ftnt. 9 in the name of R Eliyashev zt”l.