The direct reason is that water absorbs light other than blue which is scattered by the sea and upon reaching the eye we see its blue color and also it reflected the blue color of the sky.
That's the simple description. Now let's know some fun facts about the nature of light.
The white light we see is made up of very tiny particles called photons. A photon is very small, even smaller than an atom. They are present in the atmosphere but are invisible to naked eyes.
These photons exhibit both mature, wave nature as well as particle nature.
And this white light consists of various photons that have many different wavelengths, some shorter and some longer, which together make up all the colors of the rainbow. The photons with the shortest wavelength are blue and the longest wavelength is red.
So let’s discuss the sunlight. The photons that we receive from the sun and interact with are the white light. Some of them get absorbed while others are reflected or bounced back on touching the various objects. When they bounce back, this phenomenon is called “scattering”.
The photons that get scattered and are received by our eyes are what give things their color. For example, consider the leaves that are green because the green photons are reflected towards our eyes and that is what color our eyes perceive. Other colored photons are absorbed by the leaves.
Similarly, Sea appears blue and it reflects the sky too.