We have been to Curacao three times, two of the visits were in late Jan. We used to go to Cozumel during the winter, but after losing several days of diving due to cold fronts from the north we decided to try a place in the southern Caribbean to have more consistent weather. In Curacao the weather and water temps are only a few degrees cooler than in the summer months. Expect some short bursts of rain alternating with clear skies with wind. Never enough of each to cancel or compromise the number of dives.
I am the type to carry a hoodie or sweater with me when eating out, flying or going to the movies in mid July. I tend to get cold very easily. The only time in Curacao that I wore pants was when flying in and out. Much to the dismay of others, it was shorts weather the entire trip.