From San Marco di Castellabate you can reach the port close to the small island of Licosa through a short walk. It's about 3.5 km (2 miles) and it's a lovely walk to do as you're close to the coast for the most part. In some part you'll be immersed in the forest, and you'll also reach a small town at some point.
It can be done both by walking or by using a bicycle. Some cars go here as well, but not that many.
Close to Licosa, you'll also spot some small rocky beaches that are the perfect place for a swim.