Hi Shannon Marie, welcome to Cruise-Chat! The Atlantis is a great place to spend the day in Nassau. We did not do the water park, but did walk around the hotel and aquarium. There is a free beach next to the hotel, Cabbage Beach, that is very nice. We negotiated with a tour driver and he got us into the aquarium. We paid him $25/pp and he also took us to the Queen's Staircase and the fort. If you have children, the $150 may well be worth it for them, but I think they'd probably have just as much fun on the beach.
Half-Moon Cay is absolutely beautiful. Powder white sand beach, beautiful water, decent snorkeling. We haven't done the horseback ride, but I have known people that have and have enjoyed it. I used to ride, but now prefer snorkeling, less sore muscles.
We did a snorkel excursion in Grand Turk through Costa. We stopped at 2 areas, 1 for coral, the other for fish. The water was crystal clear. I don't think you could go wrong with any snorkel excursion there.
Best wishes and have a marvelous cruise!