https://www.huffpost.com/entry/stranger-things-theory-shows-bob-was-more-of-a-hero-than-you-thought_n_5a0dfa5ae4b04df7381abaef
We asked Astin about this water theory, and he proposed that Bob wouldn’t know much about it either way.
“One of the things that Bob doesn’t get to do is really experience the Upside Down. He doesn’t meet Eleven ... He doesn’t really know even in his death. He doesn’t know in the amount of time between learning about this other world and being chased down the hallway by these demodogs,” he said.
The actor claims he’s unsure whether water has a “supernatural effect” on the monsters, but Astin did bring up that his own dogs and cats react at the mere sight of a spray bottle, even though they’ve barely ever been sprayed with water.
“I don’t know if there’s anything story-wise, or within the sense of the world — that it melts them, or it’s like a vampire with sacred water on it or something ... I think for Bob in that moment, it’s like, ‘Oh, watch me freak the dogs out.’” he said.
Sure, Bob could’ve just been treating the monsters as dogs — dogs that don’t like getting sprayed with water — even though he wasn’t around when Dustin (Gaten Matarazzo) coined the phrase “demodogs.” But the evidence for water is there.
BuzzFeed asked creators Matt and Ross Duffer about the impact of water on the monsters. They said they “talked about water,” but didn’t get into the theory, adding that it was a “cool idea.”