In spring 2021, mucilage, also known as "sea snot" or "sea saliva" has been intensely observed in the Sea of Marmara and has reached a threatening level. Due to the declining water quality, many marine organisms have perished, the fishing industry and tourism have been adversely affected. In this paper, a detailed investigation was carried out to assess the effects of mucilage phenomenon on the seawater quality, sea surface temperature, and backscattered radar signal power in two separate mucilage-covered areas in the Sea of Marmara. The quality of the mucilage-covered seawater was investigated by calculating physico-chemical parameters such as sea surface temperature, electrical conductivity, the potential of hydrogen, suspended solids, dissolved oxygen concentration, and chlorophyll-a in the water samples taken. With in-situ measurements, the spectral responses of intense and middle-intense mucilage were determined by a full-range spectroradiometer and compared with the spectral signature of clean seawater. Furthermore, utilizing space-borne synthetic aperture radar (SAR) and optical images of Sentinel-1, Sentinel-2 and Sentinel-3, the effects of mucilage on spectral reflectance, radar signal backscattering, and sea surface temperature were investigated depending on its intensity. The results of in-situ measurements and laboratory analyses showed considerable effects of mucilage on water quality. The space-borne analyses demonstrated that middle-intense and intense mucilage cause approximately 0.5 and 1-decibel decrease in backscattered radar signal power against clean water. In terms of sea surface temperature, the differences between clean seawater and middle-intense and intense mucilage areas were estimated as 1.05-2.25 degrees C, respectively.