'On what do you base this rejection?'
Apart from the fact that the printed versions are the earliest known, I base it on my close studies of comparisons between multiple printings and all of the oral versions in my collections and anthologies, which are pretty comprehensive.

Equally I challenge you to name one known writer, amongst the people, of a ballad that is now found in the general oral corpus found in America and various parts of the British Isles. Once again I am not talking about local pieces only indigenous to one area, as I have already said I can give plenty of examples of these myself.

'contracted the job out'. Where is this quoted from?

I am not saying it was necessarily sailors alone. What I am saying is it is very likely the broadside hacks who were mostly from the lower levels of society itself were ex-sailors, whalermen, beggars, thieves, almost anything and had plenty of the experiences you mention. And some of them were amateur poets starting out and some were quite literate going through hard times.

I am familiar with the Jack tales of Stanley Robertson, and the American ones, and I have plenty of copies of Tocher, and I am aware of the similarities in form between the tales and the ballads, but this does not prove anything. Composing a song/poem/ballad requires quite different skills; and produces different results in the passing on of them not that this is relevant to the origins.