Had a layover there last month, and it was fine. Didn't feel the least bit unsafe to me. Especially if you're just going to be hanging by the pool (the beach sucks there IMHO, too rocky) at the hotel, you won't have any problems.
The Cancun chief of police was found executed in a ditch a few months back, but that was a drug lord's attempt at intimidating the civilized public. For what you're going to be doing, I wouldn't worry. Just take the same precautions you would anywhere, and maybe minimize venturing out into the countryside at night, or at all . . . . .
Have fun, and wear sunscreen. Cancun is about 5 miles from the sun.