Sentinel-2 and Landsat data products when combined open opportunities for capturing the dynamics of near shore coastal and inland waters at rates that have never been possible before. Recognizing the differences in their spectral and spatial sampling, to generate a seamless data record for global water quality monitoring, it is critical to quantify how well the derived data products agree under various atmospheric and aquatic conditions. This study provides an extensive quantitative assessment of how Landsat-8 and Sentinel-2A/B equivalent data products compare and discusses implications on differences in downstream products generated via the SeaWiFS Data Analysis System (SeaDAS). These products include the top-of-atmosphere (TOA) reflectance (rho(t)), the remote-sensing reflectance (R-rs), as well as biogeochemical properties, such as the total suspended solids (TSS). The analyses are conducted a) for Landsat-8 and Sentinel-2A/B near-simultaneous nadir overpasses (n-SNO) and b) over several highly turbid/eutrophic inland/nearshore waters. Following the implementation of vicarious gains for Sentinel-2A, the n-SNO analyses indicated that Landsat-8 and Sentinel-2A agree within +/- 1% in p, and +/- 5% in R-rs products across the visible and near-infrared (NIR) bands. Similar evaluations with preliminary vicarious gains for Sentinel-2B showed +/- 2% in rho(t) and 7% in R-rs products. Considering Landsat-8-derived R-rs products as a reference, we found < 5% difference in Sentinel-2A and -2B R-rs products. Analyses of combined TSS and R-rs time-series products over several aquatic systems further corroborated these results and demonstrated the remarkable value of combined products. Occasional negative retrievals of R-rs products over hypereutrophic and highly turbid waters suggest the need for improvements in the atmospheric correction procedure to empower science/application community to fully explore Landsat-Sentinel-2 products. With very similar absolute radiometric observations and products, the science community should consider developments of suitable biogeochemical algorithms to maximize the utility of merged Landsat-Sentinel-2 products.