OK This has to bring out the pedant in me .... sorry
According to the Oxford dictionary a harbour is ....a place on the coast where ships may moor in shelter, especially one protected from rough water
by piers, jetties, and other artificial structures:
I guess this definition does not exclude a natural ocurring sheltered place without jetties etc but it would include a stretch of open coastline that still provides the necessary shelter. It does not necessarily have to be an enclosed waterbody.
So if we cannot specify are harbour by its natural characteristics maybe we can define it by the number of ships it normally shelters.
On that basis I think the Singapore
and Malacca Straits are a clear winner by the several hundred (maybe even thousand) ships which are regularly moored there. I have sailed out of Singapore
and ships were moored for at least three days sailing distance. I did not record
my last sighting, I wish I had but at a guess they would have been moored for 2-300 nautical miles from Singapore and along the Malaysian coastline.
Now where else in the world can beat that????