Why did they run out of water? Surely Avraham would have given them enough to last until they reached the next inn?
Rashi: Because sick people 1 drink more than healthy people.
Rashbam: Since she lost her way, it took that much longer to reach her destination.
Targum Yonasan: As soon as she entered the desert, she began worshipping idols, and HaSh-m made Yishmael feverish, causing him to finish all the water - though his fever only got worse.
Da'as Zekenim: Refer to 21:14:2.2:1.
QUESTIONS ON RASHI
Rashi writes: "'The water from the flask ran out' - because the sick usually drink a lot." How do we know that this was the reason?
Gur Aryeh (to 21:14): Surely Avraham gave them enough water to reach an inhabited place. Rashi explained above (to 21:14) that Hagar did not become lost in the literal sense. Rashi must therefore explain that the water ran out because Yishmael was sick. However, Avraham had not realized this; he thought that Yishmael was merely overcome with emotion due to their separation.