Comment 4 for bug 1956004

Very minor comments on changelog:

+ * Closes: #1956004

While Debian uses 'Closes:', Ubuntu uses 'LP:', so this should be
  * LP: #1956004

+ * d/control: debhelper compat 13 -> 12.

technically, debhelper 13 is in focal-backports (and even bionic-backports), but if you don't actually need level 13 it's fine to reduce it in the backport.

Only other comment is that I see focal has opencpn-plugins_5.0.0+dfsg-1_all.deb, which is a dummy transitional package, but there's no corresponding opencpn-plugins package in later releases. Since it's just dummy/empty, having it installed along with the -backports version of opencpn and opecpn-data shouldn't actually cause any problem, but it was confusing for me (and I had to spend time investigating it during review) and might be confusing for end-users to see different versions of the packages installed, e.g.:

$ dpkg -l|grep opencpn
ii opencpn 5.6.0+dfsg1-1~bpo20.04.1 amd64 Open Source Chartplotter and Marine GPS Navigation Software
ii opencpn-data 5.6.0+dfsg1-1~bpo20.04.1 all Open Source Chartplotter and Marine GPS Navigation Software (data)
ii opencpn-plugins 5.0.0+dfsg-1 all Open Source Chartplotter and Marine GPS Navigation Software (transition)

Maybe the Replaces: should be updated to remove the version specifier (<< 4.8.8~)?

Otherwise, assuming you've built and tested it on focal, I see no other issues. Do you have upload rights? If not you may need to find a sponsor, or alternately you can apply to the DMB for PPU rights just for opencpn.
https://wiki.ubuntu.com/UbuntuDevelopers#PerPackage