I have written before about my work on the software doing the assembly, repair, and conversion of the OSM coastline data. I wrote the Osmcoastline software and I run it daily to create shapefiles with land and water polygons available for download on OpenStreetMapData.com [Update: Discontinued June 2019, now available from osmdata.openstreetmap.de]. For a while this data has now been used on the main OSM map. Unlike with the old process, this new process usually makes sure the map is updated within a few days when the coastline changes.