Yes, our family of 5 went the same time of year. We loved it. Especially enjoyed the "underground" snorkeling. Some of the time you are in the dark, but it's really interesting. Suggestion: get there early, go to the snorkel area first- it gets more busy as the day goes on. Wear a t-shirt under swim vest they provide. The t-shirt will keep you a little warmer. Yes, the water is a little chilly at first. On one side of the "underground" snorkeling, there is more light and some of the fish are almost clear. After snorkeling, we sunned at the extensive beach -very nice. Highly recommend this place.
It has been several years since I visited the park, but when I did it was the highlight of the cruise. Now several years later and a dozen or more cruises later, I stll think that was one of the best excursions. Be sure to do the cave rafting.