A sailing boat sank in the sea between Oropos and Eretria after colliding with the local ferry Protoporos XIII on Friday morning.
The only passenger on the sailing boat, a German national, fell into the water and was rescued by the ferry.
The ferry transported the man to the port of Eretria on the island of Evia and from there he was taken to Halkida hospital.
Port authorities will conduct an investigation into the causes of the accident.
According to local media eviathema, it was the sailing boat that collided with the ferry.