Hi Kelly,
January weather is always variable. We've cruised with seasonably warm temperatures and also with chilly days with rain. However when escaping a colder climate, any Caribbean weather is a welcome respite.
In January, I have never been able to get in the water at Castaway Cay except to wade in during the Stingray shore excursion. Our son, however, has always enjoyed the water at Castaway Cay as well as the Mickey pool and water slide despite cooler air and/or water temperatures. The
pools aboard the Disney Magic will be heated when the water temperature drops below 75 degrees.
St. Maarten will be warmer with January temps approximately 78-83 degrees. The water is likely to be around 76.
Whether those temperatures are warm enough for your family to swim is quite subjective. I'm a wimp unless it's beastly hot or the water is bath-like!
My best advice for a January vacation whether it is on land or at sea is to plan for all types of weather!