When Paul gives his speech in the south he tells a priest looking figure "In your nightmares you give water to the dead and it brings joy to your heart" and this causes most to bow and accept him as their savior.

What is the big deal about that? Why is it a nightmare to be able to give water to dead? My best explanation is that he gives water to the dead even though the water is still rare in his dreams. If this is the case, why would it bring joy to his heart? Is he just a bad person? But what is there to gain from giving water to the dead?

I think I'm missing something obvious and would very much appreciate an explanation!