For me a visit to Otranto without a meal at "La Bella Idrusa" isn't complete. Given the location, the prices are surprisingly low, pizzas are below 10Euros and they are huge. Not the best pizza ever, but definitely enjoyable and that combines with the harbour view, and the cheap prices, make the place a real attraction.
If you are after fish, seafood, there isn't much choice, but the cozze and frittura mista can be highly recommended. The frittura is made with a choice of different fish, calamari and prawns, very yummy.
Just be aware that they don't take bookings and an hour wait time is normal in summer. But then the piazza in front of the restaurant is a beautiful place to spend time, so who cares? Or come late around midnight, when it gets a bit more quiet.