Marine monitoring constitutes one of the main thematic areas of the Sentinel mission. The Sentinel 3 OLCI (S3) sensor provides satellite data for services relevant to the ocean and land. While the spatial resolution of S3 images (300m) is suitable for most marine applications, there are some applications such as floating debris detection, suspended mater estimation, etc., that require higher resolution. To fulfil this requirement this study applies an unmixing-based data fusion technique on S3 and BRDF-corrected Sentinel 2 (S2) images and evaluates the fused data by calculating the Correlation Coefficient (CC) and the Spectral Angle Distance (SAD) indexes. Then it explores the increased monitoring capabilities of the fused image by applying improved chlorophyll-a (Chl-a) and Total Suspended Matter (TSM) algorithms, developed for satellite data. The fused image presents spectral similarity to S3 data and spatial similarity to S2 image. Consequently, the products provided by the fused image have much better resolution than those of S3 image, which enables detailed estimations of Chl-a and TSM concentrations. However, the dynamic nature of the marine environment that results in the formation of time varying patterns at sea surface, in relation to the time lag between S2 and S3 image acquisitions may locally affect the accuracy of the products in the neighborhood of these patterns. This study exploits the effective elimination of directional reflectance effects in S2 ocean images, interprets the fused image and the generated ocean products, and points out the constraints regarding the synergy of Sentinel optical data for ocean areas. Index Terms-Data fusion, spectral unmixing, marine monitoring, Sentinel, BRDF correction, chlorophyll-a, total suspended matter I.INTRODUCTION ENTINEL 3 OLCI products aim at supporting a) marine safety, where the quality of the ocean is observed, b) marine resources, where resources in the marine environment are detected and studied, c) marine and coastal environment, where it is observed how these environments are affected by natural processes or human intervention, and d) sea ice, where the behavior of frozen ocean water is examined. The 300m spatial resolution matches most of the requirements for the above applications. However, there are cases, such as floating debris detection, coastal suspended mater estimation, small scale upwelling etc., where data with higher resolution are required. Fusion of Sentinel 2 and Sentinel 3 images can address this issue for coastal ocean areas, and the Mediterranean Sea, for This study has received funding from the European Space Agency's (ESA) Discovery & Preparation Programme REACT under ESA contract No. 4000131235/20/NL/GLC. Moreover, the authors want to thank ESA for the full and open data policy.