This is an interesting question with an even more interesting answer. I stumbled upon this question when I was reading an article by Helen Czerski entitled:
‘Why isn’t the sea transparent?’
In the article, she describes beautifully how when you look over an expanse of ocean, you see two blues: the sea and the sky. We can see clearly into the sky, so why not the sea?
The answer is total internal reflection.
Without this physics phenomenon, it would nearly certainly still be impossible to see under the ocean due to the concept that when two transparent things meet, light rarely travels through both of the materials.
Thus what you are looking at in the ocean is merely a reflection of the sky. This is because the surface acts like a mirror when light hits it at shallow angles. When you stand on a beach and are looking at the ocean, you are looking at it at a shallow angle. Light goes onto the ocean in a straight line from the sun and so total internal reflection occurs because of this high angle of incidence, making the water opaque from the beach position.
This phenomenon is why you can see many beautiful sunsets in the ocean too.
Of course, some light travels through the surface and should make its way out again, but there is the same mirror effect from the underside of the ocean. Even the light that enters at steep angles cannot be seen because refraction occurs, so the light’s path is bent, so consequently you would still not be able to see far under the ocean from the beach.
Furthermore, other factors like sediment, silt and dirt all play a part in the transparency (or lack of clarity) of the ocean.
Keep your eyes open for my next post,