The sea rarely changes colour as such - what changes is the light refraction, which can appear to provide patches of different colour. Refraction is affected by th depth of the sea bed, by seaweed and other matter, and by weather changes, all of which can affect the light levels that seem to show the sea as different colours.
one more thing to mention about this, "a red tide (reddish brown)" cause of Flankton,Flanktone. contain large amount of nutriments within ocean water would let them grow, having huge numbers of these organic will make it looks red. - 'seached from internet'
it doesnt know it alls! the colour of the sea is depedent on the colour of the sky on the day, as the sky is reflected onto the surface of the water! ha ha ha ha ha! im 15 hahahahahaha! you all smell