During a typical May in Cancun average daily temperatures are at 81F with 17 of the 31 days exceeding 90 degrees.
Minimum daily temperatures run at around 77F which is obviously still very pleasant, albeit a little too hot for the evening and night time. Certainly, your hotel air-conditioning will prove a necessity at this time of year.

Weather Averages for Cancun in May

image Temperature 27°C81°F
image Low Temperature 25°C77°F
image High Temperature 31°C88°F
image Sunshine Hours 11 hrs
image Rainfall 117mm
Rainfall days 15 days
image Sea Temperature 28°C 82°F

Rainfall in Cancun during May totals 4.6 inches and falls on an average of 4 rainy days through the month. At 4.6 inches this makes May the 6th driest month of the year, so kind of in the middle. The good thing though is that rain showers tend to be sporadic but heavy, never lasting more than a few minutes and within half an hour the sun has evaporated the rain water and you’d never even know it had rained.
May can be humid. Although the humidity in Cancun varies little throughout the year when high humidity combines with high temperatures (as it invariably does in May) the heat index means that it will feel much hotter than the 90 degrees it really is.
May tourists can keep themselves busy all day long parasailing, snorkelling, and riding jet skis in the crystal clear waters just off the coast.
Likewise, they can also find a spot on the powdery white sands of one of the countless beaches in Cancun.
When the sun sets, Cancun has an exciting and varied nightlife which is a constant draw for younger tourists. Many tourists come to Cancun because they want to party all night and relax all day.