We have sailed twice to the western route on other cruise lines in January. It will be cooler than what you would expect when you leave but you never know. As a Floridian told us, one day could be 50° and the next in the 80's. When we were at Key West the high the first time was 61°, not exactly swimming weather but warmer than what we have here in Iowa for that time of year. I stll wore my shorts! The closer we got to Cozumel the hotter it got and it was wonderful, shorts, swimsuits just what one would expect. Seas were calm, and I don't remember any rain storms. January is a great time to sail the western route. I have been to Cozumel in May, it was beastly hot and humid. Although I still had a fantastic time, I was on a cruiseship! happy cruising Kathy