I visited the Cancun area during the last week of October, into the first week of November. The weather was perfect. I had luckily arrived at the end of the hurricane season, and just before the heaviest of tourist seasons.
One should be cautious about heading to coastal Yucatan during the hurricane season, as the storms do blow in from the Caribbean, as they head Northward up the Gulf.
Summer is also a less than ideal time to visit, unless you are a big fan of heat.
If you can catch the beginning or tail ends of either low season, you'll get great rates and great weather.
Whatever shall I wear?
This region is a pretty casual place. Just be sure to pack for summer.
If you plan to go out for dinner or to the clubs for dancing, something nicer is in order. FYI, the club scene is hot, in many ways.
When it comes to touring around, you'll only need to be careful if you plan to enter one of the many colonial era churches in the region. They are largely Catholic, and you'll need to wear clothing that covers shoulders, elbows, and knees out of respect. Otherwise, they may not let you in.
If you plan to do any snorkeling or scuba diving and own your own equipment, it's definitely wise to bring it along. Prices can be high, and I'm sure you trust the cleanliness of your own equipment.