Hi WanMil,
The generation of the sea polygons with --generate-sea=multipolygon,no-sea-sectors sometimes still produce flooded areas.
What are the sea sectors good for, and why are you disabling them?
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.
Am I right assuming that mkgmap is generating multipolygons for sea regions? I downloaded some of the map area covered by your GPX file, and not all coastlines of islands were part of a multipolygon relation.
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
Could mkgmap ignore the natural=land when generating the multipolygon?
Hope these two things help to reduce the flooding.
How should we proceed? I can devote some time to fixing broken multipolygons if mkgmap gives me good diagnostic messages. I have already fixed all the roundabouts and turn restrictions that mkgmap complained about in Finland, over the course of some weeks. Marko