People often mistakenly think that the sea is blue because it reflects the sky, but the truth is that pure water is very slightly blue. It is just so pale that in small quantities it appears clear.
Fundamentally, what makes water blue is the fact that it absorbs light that is red, yellow and green, and scatters light that is blue ─ this is because blue light travels as shorter, smaller waves that are scattered more easily (this is essentially the same reason why the sky is also blue). The reason a small amount of water appears clear is because not much light is scattered. In larger bodies of water there are more water molecules for light to collide with, resulting in more blue light being scattered. Think of it like throwing a bucket-full of tennis balls against a tree from a distance: the chances are that most balls will miss the tree and keep going. But if you stand in a forest and throw tennis balls, more balls will hit trees and bounce back. It is exactly the same when it comes to light hitting water molecules: if there are more water molecules in the way, more light particles will collide with them and be scattered, making the water appear a deeper shade of blue.
In survival situations people often talk about the rule of three. It is said that humans can only survive for 3 minutes without air, 3 hours without heat, 3 days without water and 3 weeks without food. Not wanting to be left out, it seems that some phytoplankton also obey a rule of 3 according to a 
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