POSEIDON is an operational marine monitoring, forecasting and information system for the Greek Seas. The observing component is a distributed infrastructure made by three coastal buoys (Saronikos buoy-SB, Heraklion Coastal Buoy-HCB and Athos buoy-AB) and one Ferrybox (PFB). A calibration laboratory) is supporting the observing activities. The Ferrybox is installed on board H/S/F “Knossos Palace” and is equipped with sensors measuring T, C/S, DO, pH, fluorescence and turbidity. It is the only Ferry Box in the Mediterranean operating daily along the route Heraklion – Piraeus.