Just returned from a one week stay at the GBP (Feb 20 -27). We were disappointed in the snorkeling. The red flag was up every day for the week we were there. We attempted to snorkel anyway but only saw a fish here and there. The water overall (even around the whales) was too rough making it quite cloudly on a daily basis. Additionally, the current was very strong and we started getting pulled out by the waves on several occassions.
Bottom line, from our 1 week experience, I would not recommend the GBP for those who wish to snorkel off of a resort beach.
We did hear Akumal Bay (a short collectivo ride away) did not have the same rough conditions but we did not check it out personally. We instead went to Xel-Ha to get a snorkeling fix.