The European Space Agency's Sentinel-2A mission with the MultiSpectral Instrument (MSI) onboard was launched in 2015, initiating a new era in high-to-moderate-resolution (i.e., 10 to 60 m) imaging of Earth's resources. This manuscript describes the implementations of MSI processing into the SeaWiFS Data Analysis System (SeaDAS) and provides qualitative and quantitative analyses of remote sensing reflectance products (Rrs), which are essential in the retrievals of near-surface concentrations of water constituents in aquatic systems. In situ validations and intercomparisons of MSI-derived 11 products with those derived from Landsat-8's Operational Land Imager (OLI) both indicated reasonable products in coastal/inland waters. Following vicarious calibrations using reference in situ water-leaving radiances, the overall absolute relative differences and the root mean squared differences (RMSD) found for the matchup analyses were, < 7% and < 0.0012 1/sr, respectively, for the blue and green bands. With preliminary indications of consistency with the OLI products and very good agreements with in situ data, a time-series plot of total suspended solids (TSS) product derived from both missions was produced and analyzed for an inland system. It is surmised that frequent moderate-to-high resolution R products from the combined Sentinel-2A (and 13) and Landsat-8 missions are now available to the science/user community for developing algorithms suited for coastal/inland waters. Nonetheless, further research needs to be dedicated to a) improving atmospheric corrections over bodies of waters rich in dissolved organic matter or suspended particles, b) mitigating the impact of haze-or sea surface-reflected solar radiations at low solar zenith angles, and c) minimizing image artifacts to maximize the use of multi-mission products.