The best beach near Rome in terms of travel time, comfort and overall experience, is most certainly Santa Marinella. This lovely beach is located in a quaint bay and has clear water, a great little hole-in-the-wall for a quick takeaway lunch (beneath the Gigi Bar on the boardwalk) and plenty of lounge chairs. There is a small public area to lay out your towel, but space fills up quickly, so plan to arrive early.
How to get here: Take the train from any major station in Rome (Roma Termini, Ostiense, Trastevere or San Pietro) to Santa Marinella. The walk from the station to the beach takes about five to 10 minutes.