I'm guessing you will also want to add the new unit itself.
So will have to add the vesselname.txt and the vesselname_destription.text to the relevant folders.
It will also need sonar profile image and recognition image (otherwise it shows up as blanks).
If you use new sensors on it, then those sensors will need added into the sensors.txt file and also sensor_display_names.txt file.
The new (or edited) vessel file needs to be included in the relevant mission file that you wish it to work in too.
And also added to \override\campaign\maps\norwegian_sea_traffic.txt so that it will show up in the ingame ref and sonar library.
Seems like a lot, but its not really once you get used to it
