Been wondering how to pack for a Jan. 7th trip on the Mexican Riviera. Would love to snorkle and sun but don't know if it will be too chilly.

You can check the past temps for a city at http://www.wunderground.com/history/airport/MMAA/2005/1/12/MonthlyHistory.html

I didn't notice water temps but if the air is 80's I suspect it will be warm.

It will be warm. The first day was you go down before you get to Cabo will be chilly, but once in Cabo and then further south you will have temps in the mid 80's to low 90's. Pack shorts and short sleeves, but take a light sweater for on the ship at night, as it does get cool out on deck. Also, remeber that if you are going to be going into any of the Churches that you cannot wear shorts or sleeveless shirts. Have a great time. If you have any other questions, feel free to ask. I have done the Riviera three times now.

Sheila