@Dominions Son
Hmm, would that mean that in the modern era a fire hose or water cannon would be a viable anti-vampire weapon?
No. It does not "harm" them, no more than a holy symbol would. They just avoid it, and according to most traditions they are incapable of swimming and it weakens them. Being sprayed or rained on, no real effect.
And interesting side note, many believe that the legend possibly comes from rabies. As many of the traditions attributed to vampires are the same as if somebody had rabies. Avoiding light, avoiding water, spreading the disease to others through a bite, as well as the aggression, strength, and strong sexual desire. And becoming more "feral" as the disease ravages their brain.
In the early stages, a person infected with rabies is little different than before they got the disease. But as it advances, the biggest effect is in their behavior. Delirium, abnormal behavior, aggression, hallucinations, and insomnia (along with increased sensitivity to light means they are more active at night when others are asleep). Also, one of the other signs of the disease, hydrophobia. Hence, the origin of this aspect of the legend.