Just keep checking the internet for weather in Houston. It is pretty accurate. We have cruised from Galveston 2003 the week before Christmas. It was little chilly first day. Next morning, was sunny and began to warm up enough that people were in the pool and wearing shorts. It was plenty warm by time we got to Cozumel, but too windy for most of the water activities. Since our shore trip was cancelled, we just chose to shop.
Last year we went out of New Orleans and same was true then. We did a ruins tour in Costa Maya and it was plenty warm! If you do this, be sure to take water and a hat. And trust me there is PLENTLY of that moderate activity. I made it thru, but could hardly walk next day in Cozumel as my knees were really talking to me.