The bathymetry can relatively easy be extracted through e.g. Mapproxy or QGIS. However, you need some knowledge about geoservices/programming. If you want to give it a try, probably easiest solution is to load the WMS (Web Map Service) into QGIS. Then save it as mbtiles in QGIS. There are simple tutorials for both these steps. Here is the
you need for loading it in QGIS. Adding contour lines or soundings is more challenging but possible, with some more time..