In the case of a continent flooding, you’re all - especially the more experienced users - called upon to fix it. That’s a case where I consider it justified to make an edit in another user’s territory. It can be quite difficult however to find the exact location of the problem. To help with that, I created the following page which displays nodes where the coastline is broken in some way:
(Click on a marker in that map to see more information about the related node.)
Explanation: see this article for a comprehensive description of the requirements for coastline data. It follows that any node in a coastline way must be either
Nodes that do not meet this condition are displayed on the map. Obviously they don’t always cause a rendering problem, but in the case of a major flooding you will probably find the problematic location among them.