Reached to that point and seems maybe there is more to it, namely the letter that is adquired through being an thief or maybe slum rat background aswell.
Don't know if that has any flag attached to it as it may incriminate the prince in the involvement of contaminating the waters (that could possibly lead to a lot of diffferent things aswell, wich i'l try to see if so).
I'l attach this save where it is at the point of talking to the twins in teh bathhouse: