Chances are, if you ask an adult and a child this question: the child is more likely to get it right.
Without getting into the science of it: Water is ever so slightly blue.
When water is viewed in small quantities, such as in a glass, it will appear colourless to the human eye; but the deeper the water you are observing: (such as pool, lake or ocean) the bluer it will appear.
Yeah, but what about…!?
Although it is true, to some extent that on a clear day: bodies of water will reflect a blue sky, this only adds to the intrinsic colour of water.
Take a look at the image (above) of the bucket of water in a swimming pool, the small amount in the bucket doesn’t look very blue, whereas the larger volume of water in the pool appears a nice deep shade of blue. Both have white behind them, and are reflecting the same sky.
Also, an indoor pool with white tiles beneath (and above) will still appear blue.
For a more in-depth look at the colour of water, see the Wikipedia article on this matter