Reason 2
The second reason the sea is blue-green is due to reflection from the sky (YourDictionary). The water droplets of the sea act mirror-like and reflect the blue color of the sky. This is why if it's overcast, the sea won't look as blue, maybe even gray (Hari). The green in the water is due to the plant life underwater. The green pigment the give off reflects green light and adds the green in the blue-green colored sea (Hari).
"Image from Joost J. Bakker IJmuiden"