Just an update in case anyone tries this at some point..
I've downloaded Opencpn
onto my PC (Linux mInt) & the Draw plugin is there but only in the Flatpak version. Anyway, I exported some polyline ferry
routes as a GPX file & Draw plugin doesn't really acknowledge it - it shows in the file browser but won't add to the plugin. If it's dealing with points, presumably it would struggle to make sense of polylines - Google Earth
just imports the points and tries to joint the dots but fails, so it looks like the shapes are too complex for the GPX format? I need to investigate the GPX options more as I haven't really used this format other than importing GPS
The other option is to export a georeferenced image/create a KAP I think it is?