I've just started using the --generate-sea option, and it almost works. The problem is that of the UK, the whole of northern Scotland (north of a line through Stirling) is inverted - i.e. the land is blue and the sea is light yellow (black at night). This is inconvenient, as that's where I live. What's the likely cause? Could it be due to an island that's tagged in the wrong direction? I've checked the most northerly and most westerly islands and they all look okay to me. GeoFabrik's OSM Inspector doesn't flag up any coastline errors in that area, so that's no help. I could probably dig out the splitter log if that's any use - on the screen, this problem does seem to affect more than one of the output files - but it does look suspicious that the affected files are all contiguous.