Did you water during the day when the temperature was hot and the sun strong? If so, they are possibly burned. The water acts as a magnifying glass and intensifies the effect of the sun.

Only water in the early morning ( 7:00 - 9:00a.m.)to avoid the intense sun, and early evening (7:00 - 9:00p.m.), thus giving the leaves a chance to dry before night, and thus avoiding the development of mold.

I think I watered right before 10am (we are allowed to water between midnight and 10am every other day here).
Dahlias looked droopy from the night before (usually they perk up by the morning, but not this time). So, I have decided to give them a drink. It was hard not to wet the foliage. I guess the leaves got burned.
Thank you Jroot for answering.
The good news is new foliage is coming up, so I did not kill it completely, as I thought.
The lesson is learned!
~Natalie

Droopy look means the plant is thirsty. Do not wet the foliage, water to the roots.