Hi Charlee,
I may be listed as a junior member because I am new here. But I am not new to Cancun. I have been going there an average of 2+x per year for over 10 years. I was just there in November and they were starting construction and repairs during the slow season.
January is a slower, more laid back or quiet month. Around New Years there is a slight increase in occupancy mostly from "locals" however, that dies off as the month goes on.
Daytime you are looking at 80-85 and say 65 for evening temps. Water temps are maybe 78-80. It can get a little nippy above water with the occasional wind but not unbearable.
You can rent wetsuits when you are there relatively cheap.
You should get some really good deals. Where are you staying? I would be more than happy to suggest places to go, things to do or put you in touch with a friend of mine who runs a transportation / van service. He is a very knowledgable (and reasonably priced) guide as well as a marine biologist who gave it all up to enjoy the good life.