Why the color of the water in the sea is different from the water in the glass which has a blue paper over it?

Question

Why the color of the water in the sea is different from the water in the glass which has a blue paper over it?

Answer 1

Only 4% of the light from the blue sky is reflected from the surface of the sea.  The rest of the light goes into the deep water of the sea and is absorbed.  Light from the bottom of the sea is absorbed and doesn't reach the surface and so is black.  You see the blue reflected from the surface because you cannot see the bottom.  The water in the sea is not colored blue - the surface acts as a mirror to reflect the sky.

A glass of water is not deep enough for the light from the room to be reduced so you can only see the blue paper reflected from its surface.

Answer 2

Water itself is colorless.  What gives it color is organisms and chemicals within the water.  For instance, the occurance of a Red Tide (bloom of red algae) gives the water a red color.  Predominant green or brown algae shows green or brown water.  Lakes and ponds which have more sedimentary tendencies are darker brown due to the sand in the water and on the floor of those waterbodies.

The (presumably) tap-water in a glass has much of the impurities removed (there are some minerals and other chemicals in standard drinking water, versus distilled water), which eliminates most of the color reflection from within.  What you are seeing, the blue color is a combination of reflection of the blue paper on the water surface, refraction of light in the water itself, and reflection/refraction of the blue color interacting with the vessel (I presume here you mean it's in a glass container) itself.  The blue reflection/refraction with the water is similar to what occurs in the tropics, where the sunlight is nearly direct, and can pierce the water better.  Further away from the equator, much of the visible light is reflected off at an angle, or refracted and diffused by particles in the ocean.

I hope that helps.