This is one of my favourite little pieces of science because it is the kind of question that a small child might ask and for which there is a very simple answer that you’ll almost never get from people who know why.
Most answers you might hear will talk about reflection of the sky and the differences in refraction by the surface of the water between the red and the blue ends end of the spectrum of light.
But a simple and satisfying answer (particularly for a 5 year old child) is because water is blue, as can be seen in these amazingly beautiful photographs.