![alt text][1]There appears to be two polygons around the lake one tagged lake the other without any tags,which I think may be the problem. I would be reluctant to delete the blank one as it would been difficult to re-do. I use potlatch2 but I believe it is easier to retain the polygon on your PC to put back if necessary. necessary if you are using JOSM. Hopefully a more experienced mapper will know the safe procedure. here is the blank polygon.
[1]: http://help.openstreetmap.org/upfiles/blank_polygon.JPG