Both the sky and the ocean are blue, but for two very different reasons.

The sky is blue because of something called Rayleigh scattering. For very small particles such as individual molecules, the wavelength of light that is reflected is a function of the particle size and type of molecular bonds between atoms. It turns out that air molecules are just the right size and the bonds vibrate in such a way that they scatter blue light the most. This causes the sky to be overwhelmed with blue light.

When you watch the sky turn different colors at sunset, you are watching light become increasingly scattered by more atmosphere and larger particles (dust) as the sun descends toward the horizon.

On cloudy or foggy days, the water droplets in the atmosphere both absorb the light and scatter all wavelengths equally, causing a grey or white sky.

The ocean is not blue because of scattering. Instead, the ocean is blue (as is pure water) because of selective absorption. Liquid water absorbs preferentially both short wavelength light (UV) and longer wavelength visible and infrared light. If you drop a light sensor that measures the amount of light at different wavelengths into the ocean or into a lake, what you find is that almost all of the UV (280-400nm) and infrared (heat) wavelengths are absorbed.

Of the visible light, the longest visible wavelengths are absorbed most (red-orange, 600-700nm), then the medium-length visible wavelengths (yellow-green, 500-600nm) and last the short visible wavelengths (blue-violet, 400-500nm). This explains why very pure water, without many particles, appears deep blue. It also explains why, as you descend with depth in the ocean and the light has to travel further and further though the water, light will weaken and appear more and more blue (you may notice this diving or when looking at underwater video or pictures).

Particles and dissolved substances, depending on their size and physical properties (including the type of molecular bonds), will scatter or absorb visible light at different wavelengths and actually change the color of the water. Tropical water, which has few particles and very little color- absorbing dissolved substances, appears deep blue.

Water in very productive areas of the ocean with lots of algae will appear green. (James Joyce described the Irish Sea as "snot green" in Ulysses.) Water with a lot of sand or suspended particles will appear brown, and often river water, which has lots of color-absorbing dissolved compounds and suspended mud, will appear yellow-brown.

Scientists actually use this information to study the ocean from space-- by measuring the wavelengths of light reflected from the surface of the ocean using satellites. In this way they can measure temperature and estimate the amount and kind of particles suspended in the water.

Answer to why the grass is green?:
The simplest answer to why plants are green is that the parts that are green (e.g., leaves and stems) are reflecting green light. Objects appear certain colors to us because they reflect those colors and absorb every other color.

For example, red things reflect red but absorb everything else. (What colors do you think white objects reflect and absorb? What about black objects? ) Molecules that absorb colors are usually called pigments. We have pigments in our skin, as do most animals. Plants have many different kinds of pigments. The most common pigment in plants is called chlorophyll-a, which happens to absorb red light and reflect green. Plants need to absorb light because they use energy from light to turn carbon dioxide into proteins and carbohydrates. This is called photosynthesis. (When we make proteins and carbohydrates, we have to rebuild them from the digested proteins and carbohydrates in our food. This is called anabolism. )

Plants also need water for photosynthesis. Water is made up of two hydrogen atoms and one oxygen atom. When plants use water to turn carbon dioxide into proteins and carbohydrates, water becomes split apart into its separate hydrogen and oxygen atoms. One molecule of oxygen gas (which is made of two oxygen atoms) is generated for every two molecules of water that are split. The oxygen gas is not needed by the plant and so it is released into the atmosphere. The hydrogen atoms are conserved within the plant and are used to make energy (ATP).

Why do you think many land plants would evolve to absorb one color of light in particular (in this case, red)?

Not all plants are green. For example, some of the single-celled plants that float in the surface waters of the ocean are brown or orange. Why do you think this is? (As a hint, water absorbs some colors more than others, so sunlight actually changes once it enters the ocean).