I’ve implemented an initial version of algorithm for sea polygon rendering in Kosmos. This initial version detects if the OSM data consists solely of islands (i.e. closed “natural = coastline” ways) and in that case fills the whole map with the blue color of the sea and then draws polygons for islands.

Below you can see a sample rendering of Dutch island of Texel. The data actually consists of 3 separate islands. Compare this with the previous version. I think it looks much better :)

The sea rendering feature will be available in the next release of Kosmos.

An example of sea rendering in Kosmos