Where do those ships go when they have outlasted their usefulness? That strange, fascinating question is at the heart of “Iron Crows,†a startlingly beautiful documentary by Bong-Nam Park that is also devastatingly sad. Mr. Park, a South Korean filmmaker armed with a high-definition digital camera, a knack for striking visual compositions and a deep reservoir of human sympathy, spent months with a group of shipbreakers in Chittagong, a city in Bangladesh where many of the world’s largest vessels are taken apart, their fixtures sold off and their bodies cut into scrap metal. The workers, men and teenagers from impoverished rural villages, wade across tidal mudflats toward the grounded ships and painstakingly dismantle them with blowtorches, grappling hooks, winches and hammers. The work is dangerous, and the pay is low