The Water Fight
The Water Fight is a side quest in Dying Light 2 Stay Human.
Walkthrough
While in northwestern Horseshoe two survivors, Patricia and Maria can be found talking to one another. In conversation, Patricia will state the two got their water ration, but that Maria wants to use all of it to wash her hair. Patricia is against it, as she wants to use the water for their plants so they do not starve to death. Maria counters by stating she is sure it will rain that night, and that the combined water from the rain and the ration is too much for the plants.
Aiden has two options, either asking what if it does not rain, or stating the two can reach an agreement.
No rain
If chosen to ask what if it does not rain, Maria will just state that she is sure that it will. Patricia counters by questioning when Maria became the "local weather girl". The two will refuse to come to an agreement and Aiden will leave the conversation, completing the quest.
Agreement
If chosen to ask if the two can reach an agreement, Aiden will suggest reusing the shower water for the crops, which Patricia will refuse as soapy water can harm the crops. Maria will state that soapy water may harm the crops but germs can harm her and that while she understands she cannot shower ever week, she wants to maintain a basic level of hygiene. Two options are then give, either prioritize hygiene, or suggest using plant-based soap.
If chosen to prioritize hygiene, Aiden will points out that germs can kill and hygiene is a top priority. Patricia will find the reasoning questionable, but Aiden points out that an open mind is needed to make informed decisions. Patricia will refuse to come to an agreement as she does not see the rationality behind allowing their crops to die. Aiden will then leave the conversation, completing the quest.
The alternative is suggesting plant-based soap. Aiden will state that during his travels he heard that some plants can be used as a soap base, and while not knowing which plant was used he does point out that they already have a garden. Patricia will change her tone, no longer having to worry about the damage regular soap would do to their crop and recommends the two look up the plant in question in a book that they have. Maria will thank Aiden for the help, completing the quest.