Various types of floating macroalgae and other floating matters have been reported in the global oceans and inland waters, and their remote detection has relied primarily on passive optical sensors. These sensors provide multiple spectral bands and frequent revisits, yet they all suffer from clouds. Synthetic aperture radar (SAR) imagers are active sensors that overcome this obstacle, yet their capacity in detecting macroalgae and other floating matters is generally unknown. Here, through statistical analysis and comparison of the Sentinel-2/MultiSpectral Instrument (MSI) and Sentinel-1/SAR imagery, we attempt to fill this knowledge gap. The types of floating matters considered in this study include macroalgae (Ulva Prolifera in the Yellow Sea, Sargassum horneri in the East China Sea, and Sargassum fluitans/natans in the Caribbean Sea), cyanobacteria (Microcystis, Nodularia spumigena, and Trichodesmium), dinoflagellates (green and red Noctiluca), organic matters (sea snots and brine shrimp cysts), and marine debris (driftwood). Of these, the only floating matter that can be definitively detected in Sentinel-1/SAR imagery is U. prolifera, followed by the occasional detection of S. fluitans/natans and driftwood. In all detection cases, the macroalgae features always appear in Sentinel-1/SAR imagery with positive contrast from the surrounding waters. Because of the all-weather measurements, SAR observations can therefore complement those from the optical sensors in monitoring and tracking U. prolifera and S. fluitans/natans in their respective regions.