I still don't know if weather is going to be cold for snorkeling in beginning of January?
Nobody here can tell you with absolute certainty what the weather and water temperature will be like for your cruise. Dwayne provided a link for the average temperatures, and I'm afraid that is the best information you will find.
If you want to snorkel, go ahead and do it and buy a wetsuit. It will keep you warm if the water is cool and you can ditch it if the water is warmer than expected.