They’d but now got the Mary and they’d come up round the Horn in some other ship .... when they was meaning this other ship they called her 'the old packet' and they called the Mary by her name. He says .... from what I heard, the captain and the mate of that other ship had died something sudden. One of them was Captain Jonas Fielder, and Peter Duck realised that the man calling himself Fielder was Robert Charles Bowline (PD5).
A packet boat is a small sailing boat used for pasenger, mail and freight transport, often providing a regularly scheduled service, see wikipedia:Packet boat. Generally "domestic" services, but also trans-Atlantic, and the reference to the Horn implies an ocean-goimg ship.