We also know that fire and heat seem to harm the whole spectrum of Upside Down creatures.
But water might play a role, too — and not just because of the bathtub scene. Remember when all-time greatest dude and superhero, Bob Newby, turned on the Hawkins Lab sprinklers in Episode 8? The Demodogs whimpered and ran away.
Why would they do that? They should fear nothing, they have faces MADE ENTIRELY OF RAZOR-SHARP TEETH.
We also never saw the Demogorgon interact with water during Season 1. And when Barb got pulled into the Upside Down, the pool she was sitting next to was suddenly empty.
It's not a perfect theory, of course. Will vomited that slug into the sink at the end of Season 1, and then ran the water...
...but that slug survived and became Dart in Season 2.
But when I asked Stranger Things creators, the Duffer Brothers, about the theory, they didn't discredit it entirely. "I will say we’ve talked about water," Matt Duffer said. "I’m not gonna say if it’s specifically what you’re talking about. I won’t say more than that. It’s a cool idea."