The Bnai Yisrael needed to be reminded of the intrinsic danger of the midbat through which they traveled because they themselves did not see. The ananei hakavod took care of the snakes and scorpions. It was intrinsically a land in which thirst was a major danger, because of the heat and dehydration that would occur, and there were no oases in which they could stock up on water for the next leg of their journey. Additionally, the lack of water also caused an increase of danger from the snakes and scorpions as seen in the second quote below. That is, besides thirst, the lack of water caused a the danger of the scorpions to increase and be more deadly.
For example [Yeshivat Har Etzion PARASHAT EIKEV by Rav Ezra Bick]
(http://etzion.org.il/vbm/english/parsha/45eikev.htm)
This picture of the desert is clearly depicted in the second section.
"... the great and terrible desert - snake, serpent, and scorpion, a
thirst without water." The physical conditions of the desert are that
life cannot be sustained - there is no water, and if there is
anything, it is snake, serpent and scorpion. You of course had water,
and never met a scorpion, so that YOU lacked nothing, but the desert
lacked everything. This, I think, is the proper meaning of the
repeated phrase, "which your fathers did not know." Manna is not known
to your fathers - in other words, it is not food. The difference
between wheat and manna is not one of local habit; it is not the
difference between pizza in Italy and felafel in Beirut. Manna is not
the food of Sinai - it is unknown, never known, neither to you nor to
your fathers. There was NO FOOD IN SINAI - but you ate anyway, because
human existence is not dependent on the habitat, or on one's bank, or
flocks, but on the utterances of God's mouth.
QUESTION:
Why is Hashem's leading the Jews through a wilderness with all sorts
of snakes and his bringing forth water from the rock, mentioned in the
same pasuk?
ANSWER: The Gemara (Berachot 33a) relates that in a certain city
people were being harmed by a snake. When they informed Rabbi Chanina
ben Dosa about this, he asked to be shown the snake's burrow. He put
his heel over it, and when the snake came out and bit him, it died.
According to the Jerusalem Talmud (Berachot 5:1), a spring of water
had miraculously opened under Rabbi Chanina's heel and that sealed the
fate of the snake, for when a snake bites a person, if the person
reaches water before the snake, the snake will die, but if the snake
reaches water first the person will die.
Describing the miracles Hashem performed for the Jewish people in the
wilderness, the Torah states, "Who leads you through a great and
awesome wilderness, of snakes, fiery serpents, and scorpions, and
thirst where there was no water." These conditions were extremely
dangerous since they were likely to be bitten by snakes in places
where water was not available. The Torah therefore states that Hashem
miraculously brought forth water from the rock, which provided water
instantly to any person bitten, killing the snake and saving the
person.
