For the background the program uses nautical vector-charts in the S57 standard.
A lot of those charts are free of charge.
All available charts can be found on the special App4sail server.
You can reach this server using App4sail and all charts can be downloaded in an easy way.
The charts are all in the correct format, used by App4sail.

On the internet we are
free download cards come
Dutch waterways.
Belgian inland / Port of Antwerp.
German Waterways.
Donau Germany, Austria, Croatia, Serbia, Romania.
Elbe / Vltava Czech Republic.
Garonne / Saone / Schelde / Mosel / Rhine, France.
Upper Rhine Switzerland.

Soon more charts will be available.