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You need to take this quest at the merchant's Thorin who sells alchemy ingredients. He is in the market (SW door). After you've got the quest head on to NE building with the door. Investigate a barrel there. A sea nymph will come out of it saying you need to turn back because she is undressed and needs to cover herself. After this you can find her inside this NE building.
She will tell you she is responsive for this disturbances in the floating market. But she made it due to her brother have been captured by a merchant in the bottle. You need to break into his house to get her brother and trouble stops.
Head on to Pontiakh - Inner city. Note there are 2 merchants here that sell scrolls and stuff like that. You need Western one. He resides in the building with 2 doors. you need western one. Enter it and it collect potions there on the shelves.
In the second (from entrance) hall with many shelves. Stand in front of the path into another part of the room and turn back. Tap the wall. Secret shelf with the chest should be reviled. Take it and head back to Sea Nymph. She will reward you for rescuing her brother. You also will be able to find him near her house. He will be selling you a gem for a discount price of 500 coins.
Head back to Thorin for the reward. You will get it after he will be certain disturbances finished.