I have been to Atlantis and it's incredible. I was there a week and there is so much to do...you will feel like you need a month there!
Snorkeling/diving is pretty good. The reef around the island is protected, so no fishing/collection. Worth the cost although kind of expensive I thought for the dives. Snorkeling was almost better anyways.
You are going to have wicked time!
My fiancee and I were actually going to get married there but decided on another island in the Caribbean instead
Oh and ask them about the 'pet' puffer! I forget his name now but he is a massive Porcupine that is super friendly and always around for the tourists.