In this study, the cathodic activity of biofilmed stainless steel surfaces was investigated at two exposure depths at the same location at 1,020 m and 2,020 m depth. For this purpose, a set of passive materials and sensors were exposed for 11 months in Azores, in the Atlantic Ocean. Characteristic cathodic depolarizations due to biological activity were observed in intermediary and deep water. However, a strong cathodic activity was only measured in deep water. Potential ennoblement appeared between 80 d and 200 d, depending on the exposure depth and the experimental setup used. In a given environment, the biological cathodic activity appears to be strongly related to the limiting parameter of the reaction, which can be anodic or cathodic. The biofilm sensors exposed for the first time in open, deep water appear relevant to discriminate cathodically “strongly-active” and “weakly-active” biological activity. Under cathodic control, a high current density was measured on stainless steel in deep seawater. The experimental setup used is particularly relevant as it allows determination in situ of the maximal cathodic current density.