The best eating in Mirissa is at the informal seafood restaurants lined up along the beach road, where the fish arrived from the harbour that morning. Look for the ones with a display of fresh catch on ice outside — you will be shown what is available (often tuna, barracuda, or snapper), you choose your fish and cooking method (grilled with garlic butter, in a coconut curry, simply fried with lime), and they cook it while you sit at a table with a cold Lion beer and watch the sea.
Prices are genuinely cheap. A whole grilled fish with rice, salad, and a drink is unlikely to exceed 1,000 rupees. The quality varies between establishments but the basic proposition is the same everywhere. Come around sunset for the best combination of food, atmosphere, and light.