There is terrific snorkeling in both Dominica and Curacao. In Dominica, it's a unique experience, as you snorkel above volcanic vents which release bubbles into the water. It's like snorkeling in champagne. However, there are some fabulous hikes in Dominica as well, so you'll have to decide here. It's a nature lover's paradise.
In Curacao, would definitely recommend snorkeling. There are some boat excursions, but also there's a beach club right next to the Seaquarium which has excellent snorkeling, then a really nice beach day.
Barbados has some fun wreck snorkels, or kayaking and sea turtle snorkeling.
You're on a fabulous itinerary for what you like to do. Enjoy!