Here is a link to the weather history. Cancun Weather
The high in December is 83. The water is always warm, as you are on the Carribean side of Mexico.
It may be hard to book a wedding Dec. 31, but I wouldn't necessarily rule it out, especially if you are looking into renting a hacienda. Holiday room rates are about $50/night more expensive than in the low season....just something to keep in mind.