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:
http://opengeofiction.net/util/coastline_info.html
(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
- a member of exactly one such way, either as an interior point or as its combined start and end point (if the way is closed) or
- a member of exactly two ways, where it is the start point of one and the end point of the other
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.