Went to Magan's Bay on St. Thomas, weather and water were fine, nice and warm, but not anything uncomfortably hot (we live in the south and there are times we stay out of the water when those still living up north, as I used to, will be in the water). St. Maarten was also very warm and beach weather feeling right at Great Bay Beach at the other end of the water taxi on that trip. Sea Days were fine. CC water temp was low 70's, maybe even 70, and I stayed on shore, and DS got in for two minutes maybe before getting back on the beach and enjoying a lovely day on CC w/o going in the water for the rest of the visit. And CC was too cool for swimming but a nice if breezy day during the Jan. 16th '05 sailing of the Wonder.
I think January is a great time for the Eastern. Should be fine, and no hurricane season.