The monitoring of the Danube River water quality was performed by using a complex device including dissolved oxygen (DO), pH, redox potential (ORP) and temperature measurements, installed on a passenger cruise ship travelling between Tulcea and Sulina. This ship equipped with the water monitoring station served as a mobile device during the daytime cruise along the Sulina channel and as a fixed one when the ship was docking in Tulcea or Sulina ports, including the nighttime. The values of the Eh and rH revealed that apart from the assumed dissolved oxygen, other species of peroxide types are supposed to form in the natural water by complex biologic mechanisms.