It's like swimming in nature's aquarium. Assorted tropical fish congregate in Thunderball Grotto near Staniel Cay in Exuma, Bahamas. Credit: Danita Delimont/Getty Images Avid diver and journalist Erik Trinidad writes about a snorkeling destination in the Caribbean so rich that it rivals some of his dives. As a long-time scuba diver, I typically seek out the best scuba sites when I travel to the Caribbean. But in one destination, I didn't have to strap on an air tank to experience the wonders of the local marine life. In the Exumas —the sub-archipelago of The Bahamas comprising over 300 cays southeast of Nassau—I visited a magnificent site brimming with tropical fish. Known as “Thunderball Grotto,” this secluded, dramatic cave of jagged limestone, illuminated by sun rays peering down through natural holes in the ceiling, looks like a movie set.…