It was a sharp, windy March day, but the gray water of Casco Bay glimmered green in the sun. On his lobster boat, the Pull N’ Pray, Justin Papkee scanned the surface of the ocean, searching for his buoys. But he wasn’t looking for lobster traps. Mr. Papkee was farming, not fishing: His crop, clinging to ropes beneath the cold waves, was seaweed, thousands of pounds of brownish kelp undulating under the surface. Growing at a rate of 4 to 6 inches per day for the past six months, it was nearly ready to be harvested and sent to restaurants like Blue Hill at Stone Barns, Estela, Houseman, Saint Julivert Fisherie and Luke’s Lobster in New York, and Honey Paw, Chaval and the Purple House here in Maine. He pulled a blade of kelp from his line and handed me a long, translucent strip. I took a bite, and then another, seawater running down my chin.