One of the toughest surfaces to work with is rough concrete. Rough concrete can be difficult to apply material to because of the peaks and valleys in the surface. These peaks and valleys cause a lot of drag when you are trying to apply a coating. This is not to mention that the valleys in the surface will almost always be coated thicker than the peaks because there is no way to spread material evenly.
Smoothing out the concrete to some effect will make the coating process a lot easier. The square footage coverage that a kit of Pond Shield will yield will be a lot greater on a smoother surface than a rough one.
Use an angle grinder with a flexible grinding disk to ride over the surface of the concrete. Avoid using a stiff disk as those tend to gouge the concrete and create more of a problem. A good grit would be something like 24 or 36 grit. This will effectively peel high spots down without cutting away too much concrete at a time. Remember to always use good eye and face protection.