Maine’s Clark Cove has long been a farming community. The 40-acre inlet on the Damariscotta River estuary, six miles up the state’s rocky coast, is home to numerous farms, and a diversity of crops. But what’s most notable is the way the crops are grown. Underwater. With clean, cold and nutrient-rich waters, the estuary — where freshwater and saltwater ecosystems meet — is an ideal environment for growing marine animals and plants, including oysters, mussels, clams and sea vegetables.