What is this dark shadow on the sea?

Could be a form of algae could be the clouds casting shadows, also could be fish. The depth of the water could be different too, depending on the time of the day the tide being in or out could make the difference in depth of the water to look different. Or it could be the missing oil everyone is talking about but oil floats. so there is no way to tell for sure. Also wave propagation and tide changes could affect the color that you may have not noticed before.