You do not need su privileges to run all normal function of OpenCPN.
You do need SU privileges to install plugins (once).
Why? Because plugins may be shared among multiple users, so their administration (install/delete) is an admin function.
Trust Pavel. linux permissions may seem ridiculous and annoying at first encounter, but they will save your a-- at some point, which point may be 2000 miles out in mid-Pacific, and far from an internet
BTW, this is one reason why linux users almost never have to reload the operating system from scratch. I never have, except for planned upgrades.
The command "sudo" is your best friend on Ubuntu.
Any specific problems we can help you with?