One of the most reliable spots for fish in the old town, the Grande Osteria del Porto is a no-frills trattoria near the waterfront that serves whatever came in that morning. The menu changes daily and is usually recited rather than written down: expect whole grilled fish, fried mixed seafood, and pasta dressed with sea urchin (ricci di mare) when it's in season between October and April.
The room is small and fills quickly at lunch; arrive at noon or be prepared to wait. Prices are fair for a port city restaurant. The house white is a local Verdeca, dry and sharp, and goes well with anything coming out of the kitchen. Don't mistake it for a tourist spot — the clientele is mainly local, which tells you most of what you need to know.