We were at the JW Cancun in August and the beach was fine. It was narrow but it didn't seem rocky and the sand was good. We didn't see a lot of people swimming in the ocean. I don't know if there was a reason for that. The pool was wonderful so that's where we spent our days.
If this is referring to the JW Marriott in Cancun, I was there in March 2008. The strip of beach in front of the hotel was only enough to have a single row of chairs right up against the sea wall. However, the pool area is wonderful!
I was at the JW in CanCun in June 2007 and the beach was a narrow strip then. That was right after the winter and fall 2006 hurricanes and all the damage they caused to Mexico. The riptide there is tremendous and will suck you right back in. You can see the settp drop off when you get in the water, obvious that the beach was longer and better. Wander out a little and the sand rises up again.