Are you wearing flip-flops when you interview them? Do you tell them in advance to feel free to wear flip-flops?
> but wearing informal clothing is not much of a signal at all
You just moved the goalposts. I didn't say "informal is a problem". I said "jeans and sneakers already is the lowest reasonably acceptable way to dress for an interview" and "flip-flops or anything like that would definitely fall below just underdressing". This signals that you may believe you are too good to wear for an interview anything more than you'd wear when taking out the trash. And believe it or not that's exactly what myself and so many others noticed in practice. Many of those people do actually turn out to be very good, maybe even some of the best in the team. But as I said above they were finally a net loss for any team specifically due to this attitude.
And yes, bringing your own bottle of water would be the norm if asking for one was seen the same as asking the interviewer for a pair of decent footwear ;). Is it the same?
> they could think dressing up for an interview is cheating
Wearing jeans and sneakers instead of flip-flops and trunks is not "dressing up" in anyone's book.
You're taking the most unreasonable interpretation of everything (not just as hyperbole like my gut-punching example) and moving the goalposts just to make your point.