I made a small map of the Zeeland province with the default style and --generate-sea=extend-sea-sectors,close-gaps=6000,floodblocker
Without a land-tag it looked ok, after I made some modifications of the typ file. I had to give the sea polygon the lowest draw order.
The new version you can download here: http://mijndev.openstreetmap.nl/~ligfietser/diverse/mapnik.typ_r1946.zip