Stefan,
.kap are Raster charts -
images - and not vector charts.
On raster charts the information is on the picture or it isn't.
Nothing that
OpenCPN can do about.
OSM does not know about soundings/depth. These are not nautical charts.
For the
Netherlands and
Germany you can get the official Inland
water way charts for free. For Deutsche Binnerwasserstrassen the source is ELWIS.
In the
OpenCPN manual and in integrated chart downloader you will find complementary information.
Other up-to-date nautical charts for the
Netherlands,
Germany (and hopefully soon for
Denmark and Sweden) you will find at the OpenCPN chart outlet at o-charts.org.
Not free, as these are licensed charts, but not expensive neither.
Hubert