It still takes a little tweaking to get the coastlines right. I have manually chosen the tile borders so that the coastline will end outside the tile border. Only in the Swedish/Finnish border I am using extend-sea-sectors to make up some coastline in Sweden.
This is why I collected the coastlines for all of Europe in a single file. When all European coastlines are available, you no longer have to tweak tile boundaries.
I think that it could be useful to have the tile-splitter support a set of fixed tiles that it would split itself further as needed.
I presume you would then use this so that you extract a larger region and then let the splitter generate tiles for a subset of this region? With a sufficient margin of safety between the two regions, this should work by providing the splitter with coastline data even when tiles grow due to rounding. - Bartosz