This seems like a simple request.
But for some reason, it's often not.
The "simple" request is often followed by a litany of reasons of WHY you want a glass of water. Spare your air hostesses please. We don't need to know what exact medication(s) you are taking that you need said water for. I don't need to know that you need this glass of water not for you, but for your wife, as you point to her general direction. I don't need to know that it's for your 3-year old son. I don't need to know why you didn't bring water of your own since you can't wait for the beverage service. I don't need to know why you are coughing and how long you've been sick for.
It's like if you come up with a legit reason for thirst, THEN your flight attendants will honor your request. If not, sorry, we're gonna leave you high and dehydrated. Relax...if you want water, simply ask nicely and we will provide, no explanation necessary.
This also goes for lavatory use requests when the seatbelt sign is on. I understand when nature calls, it calls. I'm not gonna make you marinate in your own excrement. Just be careful, and do your business. I don't need to know what kind of business is being done. One time, this grown woman wearing pajama jeans felt like she needed to give me all the shatty details of her bowel growl in exchange for access to the can. Cmon, TMI lady. Keep your shit to yourself.