I'm going to go WAY out on a limb here...
And probably regret it.

Notice the appearance of "transluscency" where you can see the pebbles beneath a blurred surface far left.

Notice the darkened are where the rock "looks wet."
Notice the translucency effect lower left.
It is very easy to see how a person would look at this and say "WATER!"
http://marsrovers.jpl.nasa.gov/galle..._P2553L5M1.JPG

Second image, enhanced stereo 3D From hortonheardawho.

http://farm2.static.flickr.com/1436/...f4f895dd_o.png

However, pulling myself back in from the limb- Focus (zoom in if you need to) on the UPPER LEFT. You can see the "rippling effect in what is clearly sand merging in a slight downward pointed arc. If it was water- that would'nt be like that at all.
This is probably the clearest part of the picture that indicates this is sand without the illusion of water.