Aguaviva (364 Calle Fortaleza, Old San Juan) — The name means “jellyfish,” which explains why this ultracool, ultramodern place has dim blue lighting like a tranquil ocean, and lamps shaped like jellyfish floating overhead. Elegantly groomed oysters and clams float on cracked ice along the raw bar. Eating here is like submerging oneself into the ocean. The extensive, regularly changing menu is alive with inventive ceviches, including one with mahimahi, tomato, jicama, and Sriracha hot sauce. For something more filling, try fried snapper stuffed with lobster mofongo, or shrimp in a garlic cream sauce with yuca gnocchi. You could also empty out your wallet for one of the torres del mar, or “towers of the sea.” This gravity-defying dish comes hot or cold and includes oysters, mussels, shrimp—you name it. Oh, and don’t pass up the lobster mashed potatoes. Those alone are worth the trip.