Hi WanMil,
I remember that I implemented a first version of a patch that gives real OSM ids in the coastline warnings. But I could not find it... I put that on my todo list.
Thanks, that would be great for the next time. I guess I should file a JOSM validator bug for moving the coastline checks to the WARNING or ERROR level, because I've seen and fixed several coastline errors in the past month or two. These errors were minor: coastline overlapping islands, and two islands overlapping each other (someone basically drew a duplicate island with a slightly different shape). Last time, it was easy to spot the error from the big map. I just went to the "corner" of the box of inverted land and sea, and noticed that a straight section of coastline is missing. Marko