Water by nature is blue, but only very faintly. Most of the differences you see in the color of seawater arise from what is in the seawater. In the Caribbean sea the water looks bright blue and is very clear because there is not much bacteria living in the water. Algae can make water look green and tannins, organic compounds that come from decaying vegetation can make water looks brown. As a chemist, there are many ways to determine what is in water including mass spectrometry and absorption techniques. However, the water itself all looks the same

Yes - pour iron filings into the water. The reason blue ocean is blue is because there isn't any algae in it that colors it. Green or brown oceans are colored by algae, and those algae grow better with iron - so add iron, get more algae, change the color.