Seaside is a tourist town and its restaurants know it. The Promenade and lower Broadway are lined with chowder shops, fish and chip stands, and places selling sourdough bread bowls full of clam chowder — this is Oregon coast eating at its most archetypal. Quality varies considerably; look for places with local patrons in line rather than just tourists.
The seafood case is straightforward: Dungeness crab, razor clams, and Pacific salmon when in season are the things to order. Several spots on the Prom do fresh-cooked crab by the pound. Salt-water taffy is its own food category here and there is no shame in buying a bag from one of the shops that still pulls and cuts it in the window.