"He says - what d'you say lad, will you jump up to?"

Should be "What d'ye say lad, will you jump HER too?"
The boarding master (seeing this lad as a likely target for his nefarious games) is offering the lad a nice job ashore,
but it would mean he has to "jump ship" (he has presumably signed on for the round trip New York and back).
The job entails standing at the door of some sailors dive, maybe inducing others to enter,
but after 14 (not 42 surely-must be an error handed down) nights of this, he gets drugged and ends up "shanghied"
aboard a Cape Horner - not a kind fate for a green young lad who has only crossed the pond once (inferred in first verse)
so, older and much wiser, he offers his advice to lads like he once was himself.
And you can sing the chorus as Row, - Roll, or - Ho, row, Ho bullies ho. Whichever ya fancy. Ya takes ya choice, they's all tradishinal mateys.