We asked some specialists in dream interpretation among Ahl al-Sunnah and they were unanimously agreed that the interpretation of the dream in which someone asks the dreamer for water is that he needs something from the living that will benefit him, such as making du‘aa’ for him or paying off his debt. 

What we hope for is that this dream will be a lesson for the living, that they should hasten to pay off whatever debts they owe and seek forgiveness from people to whom they owe intangible debts. The Muslim should strive to make his balance weigh heavily by doing good deeds. This dream is a reminder to the living not to be heedless about the need of their deceased family members for prayers for mercy and forgiveness for them. 

See also the answer to question no. 763, in which there was a discussion of acts of the living that will benefit the deceased. 

We ask Allah to have mercy upon our deceased loved ones and yours, and to forgive them and raise them in status. 

And Allah knows best.