Here is a short summary of a number of the reasons presented by Rishonim for why “אין מפליגין בספינה פחות מג' ימים קודם לשבת” except if is for a devar mitzva or before three days before shabbas (mostly as summarized by the Ritva)
1. Rashbam (as quoted in Ritva and Tosfos Eruvin 43a) – The issue is one of finishing a melacha before shabbas and only assur according to beis shammai. It seems that there is a dispute if he means the melacha of a goy (the seifa of our mishna) or שביתת כלים. The language of the Rashbam in Tosfos Eruvin is “דלא שרו לעשות מלאכה בע”ש אלא בכדי שיעשו מבעוד יום”. It seems from the Rosh that we are talking about שביתת כלים since he asks the following question: Since the issur is techumim and only derabanan, even בית שמאי are modeh that there is no problem, like טוענים קורות בית הבד ועיגולי הגת. However, according to the Ritva, he quotes עורות לעבדן, and mentions the goy doing melacha for the Jew (again of brining him out of the techum)
2. Rabbeinu Chananel: The issur is techum and only applies to a low boat.
3. Rif: The problem is ביטול עונג שבת as it takes a few days to get used to the water and seasickness. (Can a large cruise be allowed as I assume you barely feel that you are at sea?)
4.Baal Hamaor: going on this trip is that you place yourself in a situation of ספק סכנה and therefore it looks as if you are creating a situation on purpose of being מחלל שבת. So, three days before shabbas is called “before shabbas” and one has to have it in mind. He therefore extends to this to other scenarios as well such as going to a desert (and what Rav Meir mentioned in shiur. Does this relate to cutting your nails three days before shabbas as well?)
5. Tosfos (Ritzba -eruvin 43a): The issur is a גזירה אטו שיטה על פני המים that you may make a boat or steer such a boat yourself for 4 amos in a karmelis (the water). Since this is just a חשש and גזירה, the takana does not take effect in case of devar mitzva or leaving before three days before.
6. Ramban: the case that is assur is where 50% or more of the passengers on the boat are ישראל and since driving the boat involves a number of issurei torah like tying and untying, if the goy is doing that melacha for you on shabbas, then it is assur. However, this is only when the boat is starting close to shabbas, since if earlier then it does not look like the goy is doing melacha for the ישראל anymore
It would seem according to all these reasons (except Tosfos and maybe Rif) that you can book your cruise without a problem and thought of when it is leaving. (there is a discussion if and how to go on shabbas as well)