Definitely don't mean to add to your anguish, but the ESTA is not meant to be used as an continually ongoing entry permit. It used to be that people would go to Canada or Mexico for a day or so and then be able to re-enter the U.S. since they had left the country. AFAIK, it has changed to the point of it being necessary to actually make a return trip to your "home country" or other distant country (not Mexico, Canada, or the Caribbean) for more than just a few days before returning to the U.S. Overstaying the 90 days permitted in the ESTA may result in not being able to obtain another one.

How do I know this?
1. A friend of mine was deported upon trying to re-enter the U.S. from Canada for the second time (after the 90 days were "used up") -- assuming that she would be o.k. in that she had exited the U.S. After the deportation (she had left Seattle to go to Vancouver for only a couple of days, she was not allowed back in and was told she could never again apply for an ESTA). After waiting 5 years, she was finally able to apply for and receive a regular 6-month visa.

2. My husband is a German national, and we had the same fear of overstaying the 90 day limit .... at which point we applied for and obtained a 6-month visa for his visits. And this is in spite of the fact that we have been married for over 30 years and I am a U.S. citizen. (And, despite this, it will cost us several thousand dollars to get him a green-card.) We were told that even with the ESTA or visa, all depends on the person you face at Customs & Immigration when entering the U.S., and that no exceptions are generally allowed for overstaying by even 1 day. Since we usually spend the entire winter in Florida, previous to getting him his visa he would always exit the U.S. and fly back to Germany for at least 2 weeks, shortly before the 90 day expiration.

Wishing you good luck! Do not take any chances if you want to be able to legally return to the U.S. in the future. Check with the State Department or a local U.S. Consulate on one of the islands you are visiting.

A couple of questions: If you are already in the Islands, did you fly there from the U.S.? If so, did you receive an exit stamp in your passport? That might make a difference in the tallying of your days within the U.S. How long in total will you be in the Caribbean before boarding the MSC ship? Does the cruise end in the Caribbean or continue on to NY?