I went last year for a wedding. Didn't get a chance to do any diving, lots of stuff going on the 5 days we were there, but we did take a boat out one day and did some snorkeling in a couple coves just outside the bay and to the south. It was pretty mediocre honestly. There were a couple small schools of fish, a moray eel and a bunch of urchin. Other than that we were mostly swimming around looking for stuff that wasn't there. It might have just been where we went, but I wasn't really that impressed. There was one other group diving at one of the coves while we were there and they were only down for about 15 minutes before they got back on the boat and left. Best part was the boat ride back in. Drinking micheladas, hanging out on the catamaran and BLAMO. 2 humpback whales, momma and a baby, popped up about 50 yards between us and the shore. Both swam parallel to us for about 20 minutes. Was really cool.
My ex and I were sitting at the bar one of the nights at our hotel and met some people that were divers. They said they go to Zih to fish rather than to dive if that tells you anything.
Aside from that, really fun place. I'd love to go back and sit on the beach again.