The riverfront strip between the Broken Bridge and the railway station concentrates Dandong's best seafood restaurants, most of them operating out of the same template: tanks of live fish and shellfish at the entrance, plastic tables, and a menu written on a whiteboard. The yellow croaker (黄花鱼) and razor clams (蛏子) are the things to order; river crab from the Yalu is a particular regional speciality in autumn, typically served steamed with vinegar and ginger.
Prices are substantially lower than Dalian or Shenyang for equivalent quality. The restaurants are open from lunch through late evening; you can usually point at what you want in the tanks without needing to read the menu. Look for places with a local crowd rather than the tourist restaurants nearest the Broken Bridge, which charge a premium for the view.