The coastline has already been fixed, but they aren't updated as often in Mapnik as other features. See [here][1], [the wiki coastline page][1], and perhaps check for Coastline tagged questions elsewhere on this help site, in particular [this one][2].
[when does coastline changes appear on map][2].
[1]: http://wiki.openstreetmap.org/wiki/Coastline#Main_Mapnik_layer
[2]: http://help.openstreetmap.org/questions/276/why-do-the-changes-i-have-made-to-coastline-not-appear-on-the-map