Romans don't swim at Ostia (despite what the metro line suggests). They drive to Sperlonga, Gaeta, Sabaudia, or take the ferry to Ponza. 1-2 hours from Rome and a world away from the concrete lidos of Ostia and Fregene.
White village on a cliff, wide sandy beach below. Grotta di Tiberio (the emperor's sea cave, Odysseus sculptures). 1.5h from Rome.
Urban beach with dramatic cliff (Montagna Spaccata — the "split mountain" where the rock cracked at Christ's death, according to legend). 2h from Rome.
A dune beach facing the sea, a lake behind. Circeo National Park. 1.5h from Rome. The wild side of the Lazio coast.
Ferry from Terracina/Formia (1-2.5h). Volcanic island. Chiaia di Luna (a crescent beach under a 100m cliff — access restricted, check status). Cala Feola (natural pools).
Below the Circeo promontory. Rocky coves + a sandy beach. The cape where Circe lived (Homer). 1.5h from Rome.
Tiny volcanic island. Roman port (carved from rock). Cala Nave beach. 2h ferry from Formia. Escaped-from-time feeling.