There are a few options available to you if you fancy a swim in Nafplio.
Just past the port under the shadow of Akronafplia Castle with lovely views of Bourtzi Castle are concrete swimming platforms with ladders into the sea. Despite the setting it’s a bit unlovely and looks a bit tired.
The outside pool has seen better days!
Arvanitia Beach is the other option and it is situated at the end of the short road [where you can park] between Akronafplia and Palamidi Castles. There are changing facilities and swimming platforms and a beach area which is pebbly but it’s not exactly a beauty spot. The large abandoned building on one side of the beach does not help. The walk along coastal path around the headland to this beach from the port is lovely, if it’s open.
Karathona Beach is just outside Nafplio. The pleasant walk along the coastal path takes about 50 minutes from Arvanitia but you can also drive there and park on the beach if you have a car. The beach itself is large and sandy. At the far end there are floaty things for the kids and there are sunbeds and refreshments available.
However I think the bestest places to swim are off the path to Karathona beach where you will find a couple of delightful little coves and if you are lucky you may have it all to yourself.