The emerald waters of Costa Rica's Gulf of Papagayo are a fish watcher's delight. From flights of mantas and mobula rays to funnel clouds of jacks and snapper, the ocean teams with life. The same coastal islands and rock pinnacles that attract pelagics also hide a wealth of invertebrate treasures awaiting discovery by the keen-eyed diver. More adventure awaits offshore on the Cocos Islands, where schooling hammerheads take center stage, with sea lions, mantas, turtles and dolphins adding to the show. Ashore, the Guanacaste region is the starting point for eco adventures ranging from volcano hikes and rainforest zip-line tours to whitewater rafting.