January is the coolest month of the year in Cancun, yet still perfect beach weather. You can swim in the ocean and sunbath in the warm Caribbean sunshine. The average high temperature is 81F (27C) and the average low at night is 67F (19C). The water temperature is 81F (27C).
It is coolest in the early morning and we reach our peak highest temps in the early afternoon.
There are a few days where long pants or light jackets are appropriate, however t-shirt and shorts are worn most days.
Courtesy of www.AccuWeather.com