Technically he hasn't been called that since DC killed his publishers in 1953. At this point he's been Shazam - officially - for longer than he was Captain Marvel. About 3 times longer - he became Shazam in any way that mattered in 1974 when he got his tv show.>>118220424
Owning a ship doesn't make you a captain. Even in civil shipping, you're still required to pass exams and produce logbook evidence of minimum hours in command before you can use titles like that.
You could call yourself that just because you owned a ship or boat, but it'd be a very expensive way of doing something wrong when you could just call yourself captain for no particular reason anyway.