Does anyone know if snorkeling is good at the Grand Sunset Princess Resort in Riviera Maya?

I am not sure about the resort itself. Normally the resorts don't have anything too special in the snorkeling sense. You may see a few little fish, but that may be about it.
There are a few places in the Rivera Maya that have phenominal snorkeling. Definetly go to Xel-Ha if you want great snorkeling.
You have TONS of room to snorkel and there are fish everywhere. Some of these fish are quite big, especially more towards where the ocean meets this inlet. (The fish ignore you. You can get close, but they scurry if you go after them) If you are looking for another great experience while you are there, do the activity where you walk on the sandy floor while wearing a scuba helmet. They through fish food in as you are down there and the amount of large fish that swim all around you is supposed to be awesome! (There is an extra charge for this)
The park itself is huge. It is an all day event. You will never get through it all in one day. You can purchase tickets through your resort. However, we found that it was less expensive to buy them at the door. I guess it depends how much your transportation would be if you didn't go with your resort.
You can purchase an all inclusive package that includes food and drink or go with the standard package. (The park also requires biodegradable sunscreen. Buying this there is quite expensive $20/ bottle, so get this beforehand.)
Another park to go to is Xcaret. This was fun as well, but not as good as the previous park.
I hope this helps.