I don't think it matters much one way or the other. Strictly speaking, you could say that it is transit because you are arriving in the US and then the next day boarding a ship that will take you to another country. Your only purpose in the US is to get on that ship.
On the other hand, cruises get special treatment in immigration law. The fact that your cruise begins and ends in the US could be taken as an indication that your sole purpose in the US isn't to travel through it, but to partake of a travel service based in the US.
I would therefore follow the advice given in BritishSam's answer and answer no, because it seems safer, but it's certainly reasonable to answer yes.
If we assume that the US government has a definite position on which answer applies to your itinerary, what will happen if you give the other answer? Will the ESTA be denied as a result. Almost certainly not. Will you be denied entry to the US? Almost certainly not. Certainly, if you put "not transit" and the immigration officer believes you will be in transit, you will nonetheless be admitted. If you put "transit" and the IO believes that you are not in transit, you will still be admitted because (1) there's no separate VWP status for transit, and (2) you can't be blamed for putting the wrong answer to a question if you reasonably believed that it was correct.