We once hosted some l7 & l8 yr. old baseball players from St. Maarten. Since we were to visit St. Maarten ourselves 3 wks. later, we asked lots of questions about their weather, etc. When we asked about temperatures, they went blank & really had to think about it. They said the temperatures are similar all year round so they really didn't know just how "cold" or "hot" it got! We have been there as early as late Feb. & as late as Aug. Yes, it is somewhat hotter & definitely more humid in Aug.
However, as someone who lived in Tampa, FL for l7 years, I do not think their weather gets as cool as FL's can in the winter. Therefore, I think you should bring a light sweater or jacket (yep, the a/c on the ships can get cold), and - at the heaviest - a sweatshirt. If you'd rather not take up space w/one, they do sell them on ships, no matter what season. I have been on deck in March & needed a light jacket many times. (Esp. when it's windy.) But, esp. since it is winter in the "north", I think you will find it very warm. Have a great time!