I'm a former ski patroller, which is a kind of outdoors EMT / paramedic. Once, on a flight from Seattle to Boston (for a YC interview!) I walked to the aft bathroom only to find a young woman who had collapsed there, still prone and lethargic, with a stewardess standing over her with a look of annoyed disgust. An evaluation indicated she was probably just dehydrated, so I had the still-annoyed stewardess bring her water, which revived her. (Of course I recommended that she see a doc when we landed just in case, but I doubt she did.) After an hour or so we were able to get her back to her seat. The rest of the flight was uneventful. The only thing that never added up was the bizzare attitude of the stewardess.
Semi-conscious, if I recall correctly, covers a wide range of responsiveness. She was unable to stand and slow to respond to questions but was well-oriented w.r.t. person, place and time. Drinking water wasn't a problem. The possible danger is that they'll vomit it up after passing out, or that they have some issue that requires emergency surgery. Now you've put something in their stomach that they might vomit during surgery, which is somewhat dangerous. But she had signs of dehydration so I did it.
In retrospect I should have given her honey or the like just in case she was having her first ever diabetic shock.
No, the possible danger is that they'll aspirate the water and you'll compromise their airway. Sounds like she wasn't semi-concious though, just tired, and maybe a little lethargic.
(edited: semi-conscious -> lethargic)