You're either endorsing literal battery/assault or perhaps overlooking some context.
Challenge studies involve consenting individuals and the actual events in the article don't.
I presume GP's example of splashing water would be on unsuspecting, non-consenting people going about their daily business, as was the case with the Subway in the article.
Then the question should've been "is it ok to perform experiments on uninformed participants" or "would it have been ok if they splashed water instead".
The doesn't try to explicitly link their question to the article and instead its prose is a hypothetical.
Instead, the guy ask if its ok to splash water. I can't think of any reason why splashing water would be the line at which its no longer ok to have an experiment. I certainly can see uninformedness being a reason not to have an experiment but that's a different question and so it would get a different answer.
You should deploy critical thinking skills. Splashing water on someone is in fact assault (or more accurately battery).
But that's besides the point, which isn't very hard to understand. The point is that just because a substance or act doesn't seem harmful _to you_, doesn't mean it doesn't violate someone's rights.
Challenge studies involve consenting individuals and the actual events in the article don't.
I presume GP's example of splashing water would be on unsuspecting, non-consenting people going about their daily business, as was the case with the Subway in the article.