Naples itself has no great beaches — the urban coastline is working port, yacht marina, and rocky shoreline. For swimming, head out to the islands or the Amalfi Coast. The volcanic black-sand beaches at Maronti on Ischia are among the best; Capri has Marina Piccola and the rocky Faraglioni; the Amalfi Coast strings tiny coves along a vertical shoreline.
Closer to the city, Posillipo has a handful of small sandy coves and lidos where Neapolitans swim in summer, and Monte di Procida at Pozzuoli has a longer stretch of beach popular with locals. Both are reachable by local bus or train. For a proper beach holiday, plan on an island or a hotel on the Sorrentine peninsula.