Sometimes the simplest answer is the truth, that the donkeys did not have enough to drink. I found this drash by Rav Avigdor Nebenzahl:
Following the crossing of the sea, the narrative records,
"They went for a three-day period in the Wilderness, but they did not
find water" (Shemos 15:22).
Three days without water in the middle of the desert! And remember it
was the end of Nissan which is a period of immense heat. I still do
not know how they did it, perhaps Hashem performed some sort of
miracle, or they may have had a limited quantity of water that was
able to sustain them for the three-day period. Finally they discover
water, only to realize that it is too bitter to drink (see ibid. 23).
Perhaps the men would somehow be able to withstand it, but what about
the women and children - can we expect a father to ignore the cries of
his young ones? Perhaps one can explain to an older child that he must
have bitachon, what about the smaller ones? There were not only
children to deal with, but there were countless animals as well: "and
flock and cattle, very much livestock" (Shemos 12:38). We do not know
the total number of animals, what we do know is: "there was not a
Jewish person who did not possess ninety Libyan donkeys laden with the
silver and gold of Egypt" (Bechoros 5b). If each of the six hundred
thousand people had ninety donkeys, there were fifty-four million
donkeys in total!! Can we begin to imagine all fifty-four million
thirsty donkeys braying simultaneously: "ee aw, ee aw"?
The noise alone could drive you out of your mind! Topped by the lack
of water their screams must have been intolerable. Rabenu Bechaye
describes this succinctly: "their journey in this great and awesome
desert with their wives and children and their going for three days
without water. After three grueling days they finally spot water from
a distance only to discover as they approached the water that it was
unfit to drink: 'they came to Marah, but they could not drink the
water of Marah because they were bitter." (Shemos 15:23). How could
they not have given up? How could they keep from going out of their
minds? Is it any wonder that "the people complained against Moshe
saying: 'what shall we drink?"
By all accounts this was an awesome test for Klal Yisrael. Chazal,
however, criticize their reaction. They regard this as one of the ten
times they tested Hashem in the desert (See Erchin 15a). Why? If a
person is dying of thirst should he not ask for water? They did not
protest to Moshe "why is it that you have brought us up from Egypt"
(Bamidbar 20:5) as we find in other situations. There is nothing wrong
with asking for water, but the way to do it is to either turn to
Hashem or to ask Moshe to pray on their behalf, not as the parsha
records: "the people complained against Moshe saying 'what shall we
drink' (Shemos 15:24) ". That was the act deserving of reproach it was
there that they tested Hashem. Even in such a desperate situation we
must not lose perspective. Hashem does not lack for ways with which to
save His nation.
Shortly thereafter in Refidim, He created a spring for them, followed
by the well of Miriam at a later stage. Had Hashem wished, He could
already have created Miriam's well in Mara. We can understand the
braying of the donkeys, after all they are only donkeys but the people
should not have acted like donkeys! The proper response would have
been to pray to Hashem and to have enough bitachon that whatever He
does is for the good. Perhaps a miracle would have occurred and water
would have appeared or they would have been able to survive without
water, or Hashem would have transported them on eagle's wings to an
area containing water. Hashem does not lack for ideas on how to
provide miracles. Even if it had been decreed that they all die in the
desert this too must be viewed as being for the best. The same may be
said regarding the other trials and tribulations they experienced in
the desert the chet haegel, the meraglim, and more. They should have
spoken to Moshe or prayed using a different "melody". Their question
of "what should we drink" should have been sung in a Yom Tov tune, not
one fitting for Tisha B'Av. It is for this that Chazal criticize them.
There is a story told of a Jew who received a letter from his son who
was in a distant country. The father who could neither read nor write
went to the local butcher and asked him to read the letter. The
butcher read in a commanding and stern voice: "Father, send me the
money! Said the father is that how my son talks? I will send him a
penny! The mother, on hearing this, suggested maybe the butcher
doesn't read so well. Let's take the letter to the chazan. They took
the letter to the chazan - and in a voice full of pathos and emotion
he beseeched "Father! Send me money!" On hearing this, the father said
- "Nu, if he asks so nicely, I am prepared to send himsome money! The same may be said of Bnei Yisrael at this point. When they asked "what
shall we drink" they used the tone of the shochet and not the tone of
the chazan. It was for this reason that this incident was counted
among the ten times the Jewish people tested Hashem.
Water requirement of a fully-grown adult donkey is in the range of 18 to 35 litres per day according to the above circumstances (Fielding and Krause 1998). Given the fact that the desert is acrid and dry, let's say 32L/day for a healthy donkey multiplied by 54 (assuming the rest of the Bnei Yisrael under 20, those over 60, and the Erev Rav did not also have 90 donkeys) yields 1 billion, 728 million litres of water or 456,489,306 and a half gallons per day. If the dew or man melted half a billion gallons a day conservatively (again just for the donkeys and not the people or the rest or the livestock), it would flood the land. It very well could be that the man was super condensed or was miraculously satiable, but it seems more likely that they simply did not have enough to drink and were required to ask Hashem. This is not to say that Hashem sent them lacking, chas v'shalom, but to shed them of their remaining yeshut representative of the Mitzri culture in order to prepare them for the ultimate waters of Torah. What separated the generation in the midbar vis-à-vis the generation that actually entered Eretz Yisrael was the presence/absence of miracles. Had Hashem kept providing for them, over and above what is natural, they would have never been able to take over and till the land as Ben Gurion has stated about Sde Boker. So, this was not so much a test as it was a trial run for what was to come.