You do not need to dilute it. It acts like any fluid and will be flat on the surface. Shiny if gloss medium, flat look if matte medium. Note that, unless it's a hockey rink, natural water isn't planar on its surface outdoors. There is always some surface rippling due to air currents. You can achieve that more natural appearance by adding a second thin coat atop the first pour that has cured and is hard. You use the side of a disposable foam paint applicator to 'stipple' the added thin coat. Do this after the second coat has begun to set up, so keep an eye on it as it cures. Or, get some gel gloss medium from an arts and crafts store and paint it on the cured first pour, also thinly. Do the same thing with the foam applicator, except you can do it right away with the gel version. The gel will cure clear, so don't be alarmed at the facial cold cream whiteness of it.