[mkgmap-dev] Sea Detection

Hi, I created a map of a cutout of the europe geofabrik extract including generate sea option. All is OK, but one tile is white instead of blue because it's completely in the sea and contains no coastline. How to detect such tiles? I think this is easy: It contains no or very few data (only some ferry lines). An option for detecting such sea tiles and filling with a sea polygon would be nice. Christian