I've been trying to find sources detailing the natural geographic features most suitable for shipping harbors along a coast—primarily for international trade. So far, I've read vague reference to bathymetry and natural harbors. What other geographic conditions should affect the viability of shipping (historically) along a country coastline?
Here are some considerations:
It needs to be deep enough that you can get close to shore with a ship that extends several meters below sea level.
It needs to be protected from large waves, so a spit of land that goes around the harbor is useful.
It shouldn't be in a place where tides are large and ships will be moving up and down over the course of a day by too much to make loading and unloading difficult.
It shouldn't be in a place where winds are frequently too strong to make operations difficult.
The answer by @wolfgang_bangerth concentrates on the maritime conditions. What also is required are conditions on land that would make such locations useful as ports for international trade.
The land adjacent to such maritime locations needs to be reasonably flat and cover a large area so that holding yards can be established for goods arriving and leaving via a port and to facilitate land based transport networks, such as rail and road, in moving goods to and from ports. Steep coastlines, as one finds with fjords, are sub optimal for use as ports.