The coastline in OSM
The coastline is handled somewhat differently than most other features in OSM. To mark it ways with the tag natural=coastline are used. Those ways need to connect end-to-end to form an unbroken line around every island and every continent. And the land always has to be on the left side, the water on the right side of those ways. For details see the OSM wiki.
Processing with OSMCoastline
The world is big and all the coastlines together are very very long. So there is a lot of data. And chances are that at any moment someone somewhere is updating the coastline. And that is not always easy to do, and errors happen. But we need one unbroken line to form proper (multi)polygons of land masses (or the sea). Specifically for this task we have developed the OSMCoastline program. It assembles all the data, applies some fixes if necessary and creates the data you see on this site. For more of the inner workings refer to the OSMCoastline documentation or have a look at these blog posts. OSMCoastline has many parameters and options and you can run it yourself to create specialized output if what you see on this site is not enough for your use case.
Finding and fixing errors
As the OSMCoastline program runs to create the coastline, it will report any errors in the coastline it finds. This information is then sent on to the Coastline view of the Geofabrik OSM Inspector. You can help fixing the OSM data by checking that site and correcting any errors you see. Thanks to all the OSM users who do this!
If you can't find an error shown by the OSM Inspector in your OSM editor, chances are somebody else was faster and has already fixed it!