The major link in the public mind is the evidence - what more do we need?
There never was any "official" instrument, but why on earth wouldn't the concertina be the sailor's first choice as an instrument.
It is tough and portable - we look after our classic instruments as if they were fragile, but they are not.
It is more "moisture-friendly" than most instruments, apart from the metal ends and steel reeds (and why else would so many wood-ended, brass reeded 20 button Lachenals have been made?) In its hexagonal case, and wrapped in an oiled cloth, it could stand very bad conditions - that would destroy a fiddle, guitar etc.
I have seen the suggestion that the button accordian would have been chosen since it was cheaper, and in fact there is evidence of accordians being used, but sailors are not notorious for their thrift, & I think that the convenience of the concertina would outweigh the cost factor.
On the issue of accompanying shanties - shanties were almost never accompanied, for the simple reason that that would put a pair of hands out of action. They were almost never sung in harmony, and were sung far slower than we usually hear them now. They were also not recorded, so all these statements are second hand and unverifiable.
Forebitters were accompanied, as were dances, and shanties were often used as forebitters, and probably to dance to as well.
Putting this together, none of the recordings of shanties we hear today are in any way "authentic" - in that they don't sound the way a person aboard a sailing ship at the time would have heard them. On the other hand they do sound good, and deserve to be heard.
Many shanties do sound better unaccompanied and sung in unison, but not necessarily all. If they are to be accompanied by any instrument - why not use the best available, the ----------?
Rod