If you are still looking for an answer, Go into Water Content/Materials/WaterSurface. Make a copy of the water material you want to change (lake,ocean,river). Double click to open up the editor for the copied material. Go down to Global Vector Parameter Values and change the RGB to the color you want. Once that is done, click on your waterbody and scroll down to Rendering/Water Material and switch it out with the new material.
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