In “Dolphin Reef,” Disneynature dives under the sea to frolic with some of the planet’s most engaging animals: dolphins. Echo is a young bottlenose dolphin who can’t quite decide if it’s time to grow up and take on new responsibilities-or give in to his silly side and just have fun. Dolphin society is tricky, and the coral reef that Echo and his family call home depends on all of its inhabitants to keep it healthy. But with humpback whales, orcas, sea turtles and cuttlefish seemingly begging for his attention, Echo has a tough time resisting all that the ocean has to offer.