For a while now I have been working on OSMCoastline adding features bit by bit. (See this blog article for some background info.) OSMCoastline now closes small gaps in the coastline and it can split up the huge polygons into smaller ones. You can create land polygons or water polygons depending on your needs and you can have it all in WGS84 or in the usual Mercator projection used for tiles web maps (EPSG 3857). So it does a lot more than the coastcheck application used for these things before. And the software is still a lot faster.