The usage of anchorage in your example is incorrect. The words anchorage and anchoring are not interchangeable. One is an action, the other a location or place.
- an area that is suitable for a ship to anchor in.
- (historical) an anchorite's dwelling place.
Now, substitute the word anchorage with the area:
Local waves within the harbor make the area difficult, if not impossible.
As you can see, that sentence is incomplete.
Local waves within the harbor make anchoring|fishing|swimming difficult, if not impossible.
An action can be made more difficult, but not a place/location.