This dataset contains Secchi-depth measurements conducted manually in association with ROV dives during the MOSAiC expedition. A 20 cm black and white Secchi disk was lowered through the ROV access hole and the depth noted at which it disappeared. Data quality of manual observations is indicated using a quality flag with values from 1 representing robust measurements to 3 indicating low data quality. The measurement rope had a length of 20 m, thus true Secchi-depth values larger than 20 m could not be measured (quality flag = 2). During the dark season, Secchi depth was determined using headlamps for illumination resulting in less robust readings (quality flag = 3). Measurements were conducted by various members of the ROV deployment team. ** For all details see the full metadata description at "https://doi.pangaea.de/10.1594/PANGAEA.929535"!