Hi Nico
In case this helps, on my Samsung tablet, the file "nautical.render.xml" is in SD card/Android/data/net.osmand.plus/files/rendering .
On my Motorola phone, the file "nautical.render.xml" is in Internal Shared storage/Android/data/net.osmand.plus/files/rendering .
PS - I'm not an expert, just an IT literate enthusiastic OsmAnd user/cyclist/walker/sailor. For serious nautical purposes I use other apps.
Paul W