What is the problem of going on a boat (and by proxy doing elective surgery) less than three days beforehand?
The Talmud, Shabbos 19a, states:
There are I believe nine different explanations in the Rishonim about why this is (and hence, in what situations it applies). As you've mentioned, one explanation is that you shouldn't be seasick on Shabbos, so give yourself a few days to gain your sea legs!
Another answer is that being on a boat (and by extension, elective surgery) is putting yourself into a situation where you may need to violate Shabbos due to life-and-death circumstances. Now certainly once you're in such a situation, you are absolutely obligated to save a life; but it's less than ideal to choose to put yourself in such a situation to begin with. According to this explanation of the Mishna, however, you only need to think about "what situation am I putting myself in" if you're within three days of Shabbos. Any earlier, and it's not your responsibility. (Similarly, if the trip is needed for a mitzva purpose, there is an argument to allow it even within the 3 days, as that wins out over the halachic strong preference of don't-put-yourself-in-a-situation-where-you-may-be-obligated-to-violate-Shabbos.