Hello,
I have been playing with the new patch and found another tile where there was issues.The tile covers the southern part of lake Huron: http://www.openstreetmap.org/?minlon=-83.71582029999999&minlat=42.8027344&ma... (disregard the flood in the southeast corner in Mapnik this was due to a now fixed broken coastline).
In that case extend-sea works fine but multipolygon fails. I have been able to reproduce this on the fairly simple attached file. See also attached screenshot for the result tile as generated by mkgmap using both options.
Thanks,
N.
Another great test simple case! But: are you sure that generate-sea=polygon really works? I have tested generate-sea=multipolygon,extends-sea-sectors,close-gaps=5000 and generate-sea=polygon,extends-sea-sectors,close-gaps=5000 but both created the same result displayed on the left in your image. Please post your mkgmap options if you are sure that polygon works for you. WanMil