Sen Toku submarine.
It doesn't sink, there's no damage model on it, there's no ship sunk icon on the nav map, it doesn't add to the warship sunk count and to top all of that it doesn't submerge either, a big candidate to be removed me thinks.
Its only good use is as target practice and after this last one I've sunk but not sunk I'll be avoiding them in future, unless you do the honourable thing and fix all of its bad points of course.