Water is actually naturally blue.

Pure water has a slight blue tint the becomes increasingly viewable as thickness and volume of water increases. The color is an intrinsic property that, like with everything, comes from selective scattering and absorption of normal white light. 

Substances in water can sometimes give it a differently colored appearance. For example, it’s common for certain beaches to have more green or turquoise water due to what’s in the water. Lakes and oceans appear more blue than pools, though, and this is for a few reasons. The surface of the water, as you’ve probably heard, reflects the color of the sky, so it is indeed a contributing factor. 

This also happens with pools though, as you might know. The reason why ocean water is generally darker is due to substances in the water. Additionally, deeper water is bluer water. Why? It has to do with the availability of light. 

As available light decreases, the perceived color darkens, so what might seem to be a light blue at a body of water with shallow depth will seem like a much darker blue in bodies of water with more depth. 

(Source)