Never been to Cancun, but we recently did an all inclusive in Cozumel, and safety was never a concern there. We were off the mainland though, so a little more isolated from the issues we've all seen on the news. Downtown Cozumel is of course a tourist trap, with 5 or 6 or 7 cruise ships parked there at any time, plus a more than once per day ferry running. But, worth the ride in to at least walk the streets and have a cerveza or maybe lunch. Get a few blocks in and the whole world changes. My spanish is almost non-existent, but was able to find a pharmacy and get more sunscreen away from the tourist areas & tourist prices. Cozumel is known for its diving, and there are lots of trips/boats diving on the reefs all day long. We snorkeled along the coast while our certified dive friends went diving almost every day. Had the same experience as lmd with the resort condition and staff..always doing maintenance, cleaning, very friendly, etc.
We chatted with the local taxi drivers while riding to/from the airport, to/from town, etc..it seems the locals all love the cruise ships..~20-25,000 people per day visit, spend their money, but none of them stay long enough to stink up the joint.