The ferry can be accessed after the events in (SPOILER)Tomb of Raithwall after obtaining the (SPOILER)Dusk Shard
You must also complete the flowering cactoid hunt.After you defeat this mark you will obtain a cactus flower.Talk to Dantro(the petitioner) and he will ask you to deliver that to his wife.She's located in the South bank village in the Estersands.Deliver it to her(she's named Dantro's wife).After that re enter the area to find the elder of the village and the little boy(Tchigri) at the docks.Talk to them for a little sidequest!!!