It actually has nothing to do with the sky.
Color is all about light, and so how the ocean appears to us (sometimes turquoise, sometimes navy, and occasionally a muddy green or brown) is a direct result of the light.
Light, as we see it, is a kind of electromagnetic radiation — a segment of a larger spectrum of energy that includes radio waves on one end and gamma radiation on the other. Our ability to perceive any kind of electromagnetic radiation depends on its wavelength.
Visible light has shorter, faster waves than radio but longer, slower waves than gamma. The human eye can distinguish between many different kinds of visible light, and the effect of this is color. White light emitted by the sun is made of the entire spectrum of visible wavelengths. We register other colors when we see only a subsection of the visible spectrum.
When some of the shortest, fastest-moving wavelengths are reflected off the ocean, our eyes see them as blue.
If you pour pure water into a glass, it’s almost perfectly translucent: the whole spectrum of visible light passes through it with little-to-no obstruction. So why does water appear blue when it forms a great body like an ocean?
It’s a matter of scale. All water absorbs the slow, undulating electromagnetic waves that we see as the color red. And blue light, because of its rapid, short waves, is more apt to hit something (a particle, a molecule) and scatter in all directions, like pinballs ricocheting around an arcade game.
The planet’s seas and oceans present a variety of blues, from the jewel-like tone of the shallow, warm Caribbean to the dusky dark of the Atlantic. The relative lightness or darkness of the ocean’s blue has everything to do with depth. In shallow areas, visible light reflects back up off of the sea floor. In deep areas, there’s no reflection.
When the ocean turns more brown or more green, it’s usually because of plant-matter or sediment kicked up by a storm or draining out of a nearby river. Animals like plankton or plants like algae can also change the ocean’s apparent color.
Of course, there are few draws more powerful than a perfectly blue body of water.
Travelers in search of the bluest waters on Earth should consider Crater Lake, in Oregon, which is almost crystal clear because there are no streams or rivers to make the water turbid.
For something a bit more tropical, consider the dazzling blue waters surrounding the Maldives, which are nestled between the Indian and Arabian seas, or the stunning, gem-like waters off the coast of Belize, punctuated only by tropical fish and vibrant coral reef.