Marathonisi creates a fascinating scenario right across from Marathias.
This pebble beach next to Keri Port is really nice. The water is fascinating and nature is nearly intact in this area. The access to the beach is not the easiest, but it is worth the effort. You can rent a couple of sunbeds for 10 euros there. Turtles are always swimming around that spot and you may get lucky and have the chance to swim with them.