Fatih, over here, there are rubber concrete "stamps" that are used to make concrete look like stones. If you decide to go the concrete route, maybe the concrete that is showing above the water level could be stamped to make it look more aestetically pleasing. They also color the concrete here - they can color the concrete in the truck, that way the color is mixed thru the concrete, and they can throw colored powder on the wet surface of the concrete to color the thin top layer of concrete.
The downside to the powder is that if any concrete chips, the natural color shows.
http://www.concretenetwork.com/products-stamps-skins/