The generation of the sea polygons with --generate-sea=multipolygon,no-sea-sectors sometimes still produce flooded areas. I have checked the finnland map and found the following things: 1. Some of the inner polygons generated for the seabounds mulitpolygon sometimes are inverted. Usually the inner polygons should denote all natural=land but some point out the sea are. This causes the flooding. I cannot precisely point to lines of code where this happens. It might have to do with self intersecting polygons. I have attached a GPX of one of self intersecting and inverted ones. Open JOSM, import the GPX, convert it to data layer, tag the polygon with natural=land. - Compare it with the real map, you will see that it is inverted. - Run the validator plugin. You will see that it has 27 duplicated nodes (I don't think that this is a problem). And there is an intersection in the east on the osmosis boundary shape. This should not create a real problem for mp. Maybe some parts of the polygon are dropped. But this might point to a problem of the sea multipolygon creation. I could easily add a warning if a way intersects itself. Don't know if that's a performance issue but will test it. 2. Some islands seem to be tagged with natural=land. They are not considered in creating the sea multipolgyon. Example: http://www.openstreetmap.org/browse/way/48666101 Hope these two things help to reduce the flooding. WanMil