Hey gregdove. The website shows highs in December around 82 degrees, and lows around 70. I would imagine the water would be a bit cool, but nothing too intense especially if it's sunny during peak temperatures. However, I would love to hear from some December Cancun veterans to get some firsthand information.
https://www.tripadvisor.com/ShowTopic-g150807-i8-k298080-December_weather-Cancun_Yucatan_Peninsula.html
Water Temperature for the best places in the world. Cancun is a well known vacation destination on the coast of the Yucatan Peninsula and the center of the Riviera Maya tourist area. It’s located in the district of Benito Juarez in the north-eastern state of Quintana Roo in the southeastern part of Mexico.
http://www.water-temperature.com/cancun
The highest day temperature in December 2015 was 90°F. The lowest night temperature was 74°F. The average values for day and night temperatures during December were 88.4°F и 78.0°F respectivly. The average sea surface temperature in December 2015 for Cancun was 85.1°F. The maximum and minimum values were 84.6°F and 85.8°F respectively. You can expect various weather conditions during December. Be ready for sunny, cloudy or even gloomy days.
http://w-weather.com/Mexico/Cancun/december/
CANCUN MEXICO TEMPERATURES. Latitude: 21 degrees north Longitude: 86 degrees west. AVERAGE TEMPERATURES (F° / C° ) Data includes the year 2006 averages.
http://www.cancun.bz/temperatures.php
Water Temperature Cancun. Cancun is a well known vacation destination on the coast of the Yucatan Peninsula and the center of the Riviera Maya tourist area. It’s located in the district of Benito Juarez in the north-eastern state of Quintana Roo in the southeastern part of Mexico.
http://www.water-temperature.com/cancun
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https://www.wunderground.com/history/airport/MMUN/2009/11/8/DailyHistory.html
December is a WONDERFUL time to go, and yes, the prices can't be beat! High temperatures are generally around 80 degrees, with lows in the 60s. Water in the pools is a little chilly, but you get used to it in a hurry.
https://www.tripadvisor.com/ShowTopic-g150807-i8-k298080-December_weather-Cancun_Yucatan_Peninsula.html
No worries - it will be dry and sunny ALL the time, with low humidity. Low eighties. You can actually walk outside in daytime and not get burned to a crisp. The ocean water in December is never cool in December - or anytime in Cancun, for that matter.
https://www.tripadvisor.com/ShowTopic-g150807-i8-k298080-December_weather-Cancun_Yucatan_Peninsula.html
Thanks to its location, the weather is warm and sunny all over the year. The average maximum temperatures range between 91 to 93 degrees Fahrenheit between May and August und reach the lowest in January and February with 62 degrees Fahrenheit. The water temperature in Cancun is always warm, with comfortable 77 degrees as the lowest and warm 84 degrees as the highest. This means optimal conditions for swimming, diving or relaxing in the Caribbean Sea and for sun bathing on the wide beaches around Cancun.
http://www.water-temperature.com/cancun
Glad to hear the weather is great in December, I'll be at the Riu Cancun the week before Christmas, we got a great deal out of Philly through Funjet. Hey weatherdood, I saw your video on your stations website, very nice indeed.
https://www.tripadvisor.com/ShowTopic-g150807-i8-k298080-December_weather-Cancun_Yucatan_Peninsula.html
average temp december cancun
The average temperature of Cancun in December is 78 degrees F to 88.4 degrees F. In Cancun, the average temperature during the month december is 88.4°F  to 78.0°F.