Marathonos is a bay just north of Perdika on Aegina island. There’s a small beach in the south east corner and several restaurants ashore. However, for more atmosphere, Perdika is only about a half a mile walk away and is highly recommended for a visit.
Marathonos offers fairly good shelter, especially in the south east corner. It’s a usable retreat if you find Perdika is full. But to me, if there were a choice, I’d take Perdika every time.