eli5: Why is water clear in small amount but blue in large amount like an ocean?

Question

I thought it might be the reflection from the sky but if that was the case, why does the ocean appears more blue the deeper you go?

In: 2918

Answer 1

Open the graphic editing program of your choice, fill an area with a rich blue and set the layer’s opacity to 5%. Now duplicate that layer again and again. In my test a stack of 64 appeared the same as one layer with 96% opacity.