Optical imagery at low tide will allow identification of sandy versus muddy seabed areas; however the Wadden Sea will not be without water for its entire area at the same point in time and its parts only for a relative period of time. This tidal dynamics implies you would need to make a composite of many images at low over a large period, decades rather than years.
The muddiness of the water is not detectable by optical imagery. Moreover, the muddiness is unlikely to be correlated with the muddiness of the seabed at the same location because of currents and winds.