Given the general rabbinic focus on how exile came about, I can't help but wonder if there's any connection between these midrash and the rabbinic view of the cause of midrash. That is to say, there are a host of rabbinic commentaries in which Torah is compared to water
(in fact there's an extended analogy in the midrash Song of Songs:
The Torah has been compared to wine, water, oil, and honey and milk. Just as we find water all over the earth's surface, so do we find the Torah; water will never cease from this globe, neither will God's laws cease. Water comes from the heavens, and the Torah came from heaven. There is a noise when water descends, and the Torah descended amidst thunders. Water quickens the thirsty soul; so does the Torah quicken him who is thirsty for knowledge. Water cleanses impurities, and God's laws do the same. Water coming down by drops can form a river; so if a man acquires Torah bit by bit he may eventually become a great scholar. Water, unless one is thirsty, cannot be drunk with any degree of pleasure; in the same way, unless one has a craving for the Torah, its study, if enforced, will become a burden. Water runs from high places and seeks the lower portions of the earth; so the Torah will not remain with the haughty man, but rather seeks out the lowly. Water is not kept in golden or silver vessels, but is best kept in earthenware; so the Torah will not be retained except by him who is meek of spirit. A man of distinction will not think it beneath his dignity to ask for water from the meanest individual, neither is any one too great to despise instruction from the most insignificant person. One may drown in water if one cannot swim; so, unless one possesses a thorough knowledge of the Torah and all its meanings, one may be drowned in it. But it may be said that water gets stale if kept for a time in a vessel, and that the same should apply to the Torah. Remember therefore that it is also likened to wine, which improves with age. Again, water leaves no trace on him who tastes it, and the same, it might be said, must be the case with the Torah. But here again we must remember the comparison of the Torah to wine. just as wine has a visible effect on one who drinks it, so the studious man is at once known when one looks at him. Water does not rejoice the heart, and it might be concluded that the same is true of the Torah; hence it is likened to wine, since each rejoices the heart. Yet wine is sometimes injurious; not so the Torah, which is compared with oil. As oil is capable of anointing any part of the human body, so is the Torah an anointment to its possessor. But oil again has a bitter taste before it is purified; is this, then, equally true of the Torah? No; for the Torah is compared to milk and honey, each of which has an agreeable taste, while when blended they have healing properties as well as sweetness.--Mid. Songs 1."
and in the Talmud Baba Kama 82a Torah is compared directly to water culminating in quoting from Isaiah 55:1
"HO, all who are thirsty, come for water,
and you who have no money,
come, buy and eat!
Come, buy wine and milk
without money and without cost.
2 Why spend money on what is not bread,
and your labor on what does not satisfy?
and stating bluntly, water means nothing but Torah.
Could what the rabbis really be getting at not so much be the focus on our external enemies and betrayals, but a more internal reckoning - if you had drunk the waters of Torah when you were in Israel, perhaps now you would have water to drink, if you had eaten what truly satisfies, your enemies would not have been able to overcome you.
It is a standard trope of biblical and rabbinic literature to have exile or domination be internally caused, even when delivered by outside forces.
Why the Ishmaelites in this case? Perhaps it is because it is all the more galling to be betrayed by one's brother, and not a stranger. Although there are also echoes of turnabout, since Ishmael was, too, turned out into the wilderness by our father Abraham, and his mother thought he would die without water. Yet, he survived, and so, although in great distress, did Israel.