

That quest here to find the missing sister is just the worst:
No map marker, quest giver gives misleading directions, the ‘obvious’ cave leads you through a completely unrelated dungeon, only way to finish the quest is find an incredibly small alchemist pouch buried in mud underwater in a strong current surrounded by tall foliage, only quest rewards are alchemy ingredients.
I had correctly guessed she drowned, so I checked downstream of the thinner and shallower (waist deep) river side of the island, which was closer to the town.
But no. She drowned upstream of the island trying to cross the river at its broadest point and further out of the town, apparently.
I think Bethesda just wanted me to finally use up those potions of water breathing.