When you're not in the water, it looks lighter, like this: http://bit.ly/1xzf8VW
When you ARE in it, on the other hand, it becomes darker, and the sky darkens as well: http://bit.ly/1uMJgL5

To reproduce:
1) go to a river (to see it)
2) Go in it just to float, or on a block under a water block space.