I genuinely don’t know how to respond to this

I think the water comment might be sarcasm

Water is actually essential for the human body to function properly. Yeah, they're being sarcastic.

https://preview.redd.it/89p27uyztj2h1.jpeg?width=864&format=pjpg&auto=webp&s=2d370b3f5dd3e81c18ba7875e35f36eaf8486478

There’s some things that you really just can’t teach people lol

Just a bad response when they don't have a good response to the question

Tell them, "You can research this yourself. Just spend one week stranded in Death Valley in the middle of the summer with no water, and then get back to me with your findings-- if you survive, that is."

Yeah. If you last 40 days without it then you are either dead or you’re Jesus.

Based on the context, I'm guessing they're being sarcastic and think whatever plant this person is talking about being good for hair is as common sense as drinking water