Thanks. I'm not really sure why the coastlines don't always show up, I'd noticed it too. I think that perhaps it doesn't know that these tiles are supposed to be mixed land/sea tiles, and is instead rendering them as sea. The presence of roads apparently doesn't deter it. Try visiting informationfreeway.org and viewing this island at zoom level 12. Then hit 'x' for each tile and maybe we can convince it that it really is mixed and not sea. Of course, perhaps that only affects osmarender and not mapnik, but it's worth a try.