They're talking about the natural water erosion that happens to an island in its formation, your archway is one of those geological formations that occurs. They want to have some coves, bays, harbors, inlets, swamps, things that would happen naturally if an island had experienced a million or so years of wear and tear.
Thats what I think they mean...
aaaaa
Ive planned so many more natural formations like that, just wait till Im finish :C
