PUNTA BRITO, Nicaragua — One of the largest engineering projects in history would start here, on this desolate and pristine crescent of dark-sand Pacific beach. From in front of the hammock-swinging shrimpers on the porches of their tin-and-wood shanties, the trench would run east in sinuous curves up through the mangroves and banana fields until it reaches the shores of Central America’s largest freshwater lake, then cut across the Caribbean highlands and through indigenous territory, ending its journey 172 miles away on the beaches of the Atlantic. They call it the Grand Inter-Oceanic Canal: an audacious $50 billion plan by an...