The submitted datasets contain surface seawater partial pressure of carbon dioxide (pCO2) values measured with Kongsberg Contros/4H-Jena HydroC-FT membrane-based sensors. These sensors were integrated on the FerryBox installed on the commercial vessel Hafnia Sea (DFDS Seaways shipping company) travelling in the North Sea and maintained by the Coastal Research group at the Helmholtz-Zentrum Geesthacht, Germany. The pCO2 data were reprocessed from the raw data and corrected for sensor post-calibration where this was available. The instrument produces a result every second. 20-second averages are used for calculations and reported. Temperature and salinity results are also provided where available. These were measured from the underway with Falmouth Scientific Instruments / Teledyne Instruments sensors also integrated with the FerryBox. The sensors were regularly maintained and occasionally replaced.