I checked the guides for a place for dinner, and settled on the Trattoria Pizzeria de Pasqualino. It was great. The book says 18 euro per person for dinner, but for a total of 30 euro, including a small tip, we got a bottle of Lacrima Christi (yummy fruity red wine.) I ate a vegetable antipasto platter with sun dried tomatoes, artichoke hearts, some kind of greens, grilled eggplant, grilled zucchini, all drenched in garlicky olive oil. Mark had proschiutto with fresh mozzarella. For a pasta dish I had spaghetti pomadoro, and Mark had Farfalle with a salmon sauce. Everything was good. We did not need a further main course.
The menu allows for a full Italian meal, with appetizer, pasta course, main course, etc., or for just a light pizza. There are a number of options for vegetarians, both with cheese, as much of the pizza (you can get marianara pizza without cheese) and without, as in my antipasto, which was heavenly.
If you are staying at the Hostel at Mergellina, or you are coming via public transport to the Mergellina train station, this is easy. It's just down the street from the hostel and the station. Exiting the station, turn right. You will go under an underpass and come out to a busy square with lots of tables (several restaurants seat people here.) From here you can see the water.