The sea in serapo is quite clean, even though the beach is VERY crowded. the beach is divided into many "stabilimenti balneari" organized with umbrells and beach chairs, bars and restaurants. it is quite exensive to hire umbrellas and chairs there (i think at least 30 euros) if you move some miles further you will get to the PIANA DI SANT'AGOSTINO, a few miles before getting to sperlonga (which is a nice place to see...), well, ,the sea there used to be (and i think still is) very very clean and the beach is not that crowded. it is quite difficult to reach the beach, the shore is very high (it is called the spiaggia dei 100 gradini - 100 steps) but it worths the effort... you can also park the car "downstairs" but, be prepared it is quite expensive. i advice a place called PAPARDO' which has umbrellas, chairs and a bar. if you get there early in the morning (9.00 - 9.30 ) you can find a good parking place ( repeat quite expensive...) but it'll save you the climb 'n the wayback....the bar downstairs prepares good sandwiches and the view of the bay is amazing. there's another place a little further called AENEAS, even better organized but very expensive... avoid saturdays and sundays.. you will likely get stuck in the traffic jam before formia.... enjoy hte place!!!!