Spiaggia del Pozzillo is a large sandy beach very popular among locals. It spans for 2 km, and it connects the town of San Marco and Santa Maria di Castellabate.
Here you can find various beach resorts where you can have lunch, enjoy an ice cream or a drink, and rent umbrellas and sun beds.
The water is quite low here, so you'll likely have to walk a little to reach a spot where you can't touch.
It gets a bit crowded, especially in the free part of the beach. However, it's a nice spot for a swim and if you are in the area I'd go at least once.