If ambiance counts, then this place is number one. It's like being at an old New England clambake back in the day.
Along the coast near Rome there are lots of fine seafood restaurants, but this experience is just what the tourists crave for. To swim and then go in for lunch is great, under the shadow of the old castle on the seashore.
If you have a rental car take the A12 north of Rome towards Civitavecchia and get off at Santa Severa (park at the castle off Via Aurelia). If you take the train, take the Civitavecchia line, get off at Santa Severa and walk to the castle (this place is on the beach just north of the castle).
L'Isola del Pescatore
Via Cartagine, 1
Santa Severa (RM)