Spiaggia della Grotta in San Marco di Castellabate is a popular beach among locals, located near the port. It's a public beach, so you won't be able to rent sunbeds and umbrellas, but you can bring your own and just use a beach towel.

The beach gets crowded quickly, as it's quite small, and it's the closest to the town centre, so it's easy to reach by many.

I recommend going here around 8:30/9 am, so you'll be able to get a nice spot. An alternative would be coming around lunchtime 1/1:30 pm, as most people go back home to eat.

The best is coming here in the morning, as the water is best during that time.