I could not find an answer for this question here in the official forums. The only info I found about it was on Reddit and the answer there was wrong. So just in case someone else comes up with this question, here is the correct answer.
In the Aquarius quest, when you are talking to candidates about who will carry water, the candidate named Rock mentions his arm was hurt in the past. If you choose the correct dialogue prompt after he mentions it, you will get a sub-objective in the quest’s detailed info about helping him with his injury. Specifically, the quest wants you to find a concoction for Rock’s arm that will help heal the injury.
The question: what is this concoction?
Answer: Marigold Decoction.
They are plentiful in the world, especially as drops on bandits, but you can also make them at the Alchemy bench if you’ve learned the recipe, or else just buy one ready-made straight from the Apothecary. In order to be given the option during conversation with Rock (during the interview or later) to help with his injury, you must be carrying a Marigold Decoction in your personal inventory. Alternatively, if you have level 2 in First Aid (a Main Level perk), you will also have same dialogue option if you at least own a Marigold Decoction outside of your personal inventory (eg. on your horse).
Warning: you must complete this sub-objective before fully turning in all names to the Bailiff quest-giver, otherwise this sub-ojbective for Rock will fail.