The Ferryman's Missing sidequest is actually the Cactoid Family sidequest. It's available after getting the (SPOILER)Dawn Shard, and you must have completed the Desert Bloom Hunt and given Dantro's wife the Cactus Flower. After all that, talk to Tchigri by the boat and he'll take you across the Nebra River, and it's pretty straightforward from there. That will also give you unlimited access to the second half of the Estersand.