Hello. Well, I have never done a Mexican Rivera cruise. I have done a 4 night Baja cruise. Ensenada was as far south as we went. We went during June, 2003. The weather during the day was great, hot, with lots of pool activities. But, as soon as the sun went down it was FREEZING out on deck. We did not bring a jacket, thinking it was summer. We lived in So. California all our lives, and didn't think a jacket would be needed. The issue was the wind, it was very cold out at sea. So cold, my husband and I both spent $60 each on RCI sweatshirts with hoods, so we could hang out on deck. So, I can only assume that since you are cruising during the winter, it would be just as cold out at sea, maybe even colder. I am assuming this, since I have never taken a riviera cruise at all. However, I was in Mazatlan for 4 nights last December 1st-4th. The weather was nice enough, (fairly hot) for us to sunbath by the pool. But, as the sun started to go down, it started to get cool, that most of us would cover up with pool towels to take the edge off. My mom has gone to Mazatlan every December for 2 weeks for many years, for their timeshare. She said the weather we had is typical. So, it seemed to me the days are warm/hot, and the nights would be cold with a cold wind out at sea. I hope this helps. Enjoy your cruise, Lil' Lori