Okay, I went medieval on it.
DO NOT EVER TRY THIS AT HOME!!
The problem is circular dependencies. OpenCPN thinks it requires "opencpn-tcdata", and "opencpn-gshhs-low" before it will install. Both debs require OpenCPN to be installed before THEY will install. Catch-22
So I did a brute force hack on the ailing OpenCPN .deb file for my distro (Xenial). I unzipped it and modified the "/DEBIAN/control" file, removed those two offending dependencies, saved the file, and re-assembled the OpenCPN .deb file.
It then installed without errors, and I was able to load chart data into it. I was then able to install the "opencpn-tcdata_3.3.2107-0~utopic1_all.deb", "opencpn-gshhs-crude_2.2.4-0~utopic2_all.deb", and "opencpn-gshhs-low_2.2.4-0~utopic2_all.deb" files without errors.
Sooo .. Maybe this will help someone else. Dunno. Maybe the package maintainer will see this. Dunno ..
But anyway, it's a strange way to run a railroad, ain't it? ... Yeah, WE know ..