How we cite our quotes:
He was gentlemanly, steady, tractable, with a thorough knowledge of his duties; and in time, when yet very young, he became chief mate of a fine ship, without ever having been tested by those events of the sea that show in the light of day the inner worth of a man [...] (2.1)
From the get-go, Jim sounds like a good guy. He has all the qualities you'd want in a strapping young sailor. But those qualities don't mean anything until they're tested, do they?
"I would have trusted the deck to that youngster on the strength of a single glance, and gone to sleep with both eyes – and, by Jove! it wouldn't have been safe. There are depths of horror in the thought. He looked as genuine as a new sovereign, but there was some infernal alloy in his metal." (5.11)
Marlow's horror at this scenario reveals his real issue with Jim's behavior. Marlow would have trusted Jim based on his looks alone, but that would have been a very bad idea indeed. Jim may look like a gentleman, but aboard the Patna, he sure didn't act like one.
"In his letter to the owners – it was left open for me to see – he said that he had always done his duty by them – up to that moment – and even now he was not betraying their confidence, since he was leaving the ship to as competent a seaman as could be found [...]" (6.8)
How telling, that Brierly tries to be dutiful even in his suicide. And how sad.