Why ocean appears blue? Why ocean appear to be Blue?
I have got two answers

*

*It's surface scatter blue light more

*Water absorb red ,yellow, green  are absorbed and hence what left is bluish shades.

But ..I got confused, which is the dominant reason?
If I go underwater than "2" one is applied as what I see inside water is only blue color not red that is absorbed.
But I am talking about why I see blue if I am looking 'at' surface of sea? Is that "1" reason??
I am totally confused, please explain it from beginning and in detail (upto high school level)
 A: According to the US National Ocean Service:

The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see.
The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water.

A: What gandalf61 says is true. Another reason is the ocean reflects the color of the sky.
Here is a photo of the ocean from Why the water is blue?

The background is deeper water. It is dark blue, in particular darker than the sky. When you look in the direction of that part of the ocean, the only light you see is reflected from the sky. The surface doesn't reflect much, so you don't get very much blue light. You get no light coming from beneath the surface.
The light part is shallow. Light from the sky reaches the bottom and is reflected back up. So what you see is a combination of a little light reflected from the sky and more light reflected from the bottom.

Here is a photo of glasses of water from a random website (https://www.yourhealth.net.au/wp-content/uploads/2017/06/Glasses-of-water-on-a-wooden-table.jpg)

Like the ocean, a glass of water also changes appearance depending on how much light is reflected or transmitted from below. The surface reflects some light. For the glass on the left, most of the background light comes from the table. For the glass on the right, the angle is just right to reflect a bright light off the surface toward the camera.

Water is very nearly totally transparent to blue light, and fairly transparent to red. Blue light is absorped over hundreds of meters. (Particles in the water can change this.) Red light doesn't get past a few meters. See Why the water is blue?
One difference between the photos is even the shallow part of the ocean is much deeper than a glass. Even a white ocean bottom appears blue because light is absorbed over a long path. For a glass of water, almost all the light from below is transmitted through the glass.
A: There are multiple effects at play, and how much each contributes depends on the depth of the water, as well as on the direction you're looking at.
If you are looking straight down into the water, then the dominant effects are light absorption and light scattering. If the water is shallow, most of the light reaches the bottom, and is reflected back, so the water appears transparent - unless it's muddy or murky.
If you go further away from the shore, you'll notice that you can still see the bottom, but that its hue is a bit different: it's blue-greenish, and as you go further and further from the shore, it may appear as if you're looking at it through a bluish haze. This is because the light in red-orange-yellow part of the spectrum mostly doesn't reach the bottom - because water molecules absorb it. The blue part of the spectrum, however, partly reaches the bottom, and is partly scattered around (similar to how it's scattered in the atmosphere). The rays that are reflected off of the bottom and manage to reach your eyes is what lets you actually see the bottom. If there's a lot of scattering, the scattered rays come from random directions and thus don't really carry an image - they just produce the blue haze effect.
As you go even further away from the shore, all light is either absorbed or scattered before it reaches the bottom, and all you see is the ocean blue.
Notice also that large waves sometimes have a blue-green tint (especially near the top), even though the ocean from which they arise appears a deeper blue; this is because the thickness of the wave is not enough to eliminate all of the non-blue light.


See also: Ocean Color article from NASA Science website

But now, change your viewing angle - go from looking straight down, towards looking at the horizon. As you change the angle of your gaze, your eyes "catch" the light rays coming from lower and lower angles with respect to the ocean surface. Well, water partially reflects light, and it reflects more light if the angle of the incoming light ray is low (with respect to the surface). E.g. look at this photo - you can see that the water reflects the sky, including the clouds:

(free stock photo: source)
In the image below, you can see the lake floor through the water near the bottom of the frame, but as you go towards the other shore you generally see more and more of the reflection, until all you can see are the reflected trees. This is somewhat perturbed by the wavy surface - the waves change the angle of incidence, so in the lower left you also get a discontinuous reflection of the mountains and the sky on the parts of the wavy surface that are angled with respect to the observer.

(free stock photo: source)
This is called Fresnel reflection.

See also:
Fresnel Equations
Snell's Window

A: I think it's simply Rayleigh scattering. Intensity of scattered light is inversely proportional to wavelength to power 4. Violet indigo blue has less wavelength and more scattering power. That is same reason sky is blue .
