I was looking on the cruisecritic message board for the same questions and Dzul Ha seems like it has calm waters and is not very crowded. Look under ports on the message board there. I think I will change my plans & go to Dzul Ha instead of Chakanaab.
From what I understand, it is more of a reef than a beach. Rocky but with nice ocean life to see. If I read correctly there is a beach club across the street by the same name that isn't very expensive to get into. I think maybe $5. You can find it's web site by searching on Dzul Ha Beach Club. I also found this:http://virtual-showcase.com/wwwboard/messages/9491.html
We were snorkeling at Dzul Ha in November, snorkeling is GREAT, beach NOT! It was very quiet there not too many people, just us and 2 other couples. It is not far from the pier which is south of San Miguel too far to walk though. Yes, it is shallow you can see lots of Marine life here without having to dive down into the water.