This dataset contains paired optical and biogeochemical properties sampled in the Belgian Coastal Zone and inland waters of Flanders, Belgium, for the growth season period (March to October) of 2017, 2018, and 2019. This dataset was collected to support: (1) development and validation of hyperspectral algorithms for aquatic retrievals with in situ instrumentation; (2) development and validation of hyperspectral algorithms for aquatic retrievals with Earth Observation; (3) development and validation of multispectral algorithms and aquatic applications to high spatial resolution sensors. The data were collected over a series of short campaigns, with variable frequency depending on the water system and sampling year. The systems sampled were lakes, a lagoon, an estuary and coastal waters. The data consist of three types of data: I - Inherent Optical Properties: - Spectral CDOM absorption coefficient (< 0.45 μm; cuvette): (A) Measured, (B) Fitted to hyperbolic model, (C) Hyperbolic slope; - Spectral particle absorption coefficient (> 0.7 μm; filterpad inside integrating sphere): (A) Measured; - Spectral de-pigmented particle absorption (ad) coefficient (> 0.7 μm; chemical oxidation): (A) Measured, (B) Fitted to exponential model, (C) Exponential slope; - Spectral in vivo pigment absorption coefficient (> 0.7 μm; calculated by difference): (A) From measured ad, (B) From fitted ad; - Spectral particle scattering coefficient (acceptance angle of 0.074 degrees): (A) Measured, (B) Compensated to equivalent 0.018 degree acceptance angle; - Spectral particle beam attenuation coefficient (acceptance angle of 0.074 degrees): (A) Measured, (B) Compensated to equivalent 0.018 degree acceptance angle; - Near-forward particle volume scattering function (laser diffraction at 670 nm): (A) Measured with a LISST-100X, (B) Measured with a LISST-200X; - Turbidity (side-scattering, NIR): (A) Measured; II - Apparent Optical Properties: - Lambert-equivalent substrate bi-hemispherical reflectance (spectrometer and imaging ...