Certainly, this is possible. Almost all things are, after all.
However, the effect of melting when exposed to large amounts of liquid water is probably not what it initially appears to be.
An alien organism that is tolerant to small amounts of water is probably one that is tolerant to larger amounts too, so why would a large amount of water make the alien melt?
My hypothesis is that this particular alien is actually composed of a great many small subunits. This is similar to our cellular model, but the alien subunits may themselves be tiny, aquatic, multicellular organisms. These organisms have the ability to clump together into a single colony in times of drought in order to present a reduced surface area through which water may be lost, and when a sufficiently heavy rain comes along, they can disband and do their own thing, such as mating.
Over time, these organisms have evolved the ability to move and form a group intelligence while joined in a colony, however the single consciousness which results is dependent upon the particular arrangement of the individual organisms, and should the colony disband, its consciousness will cease, and even if the colony subsequently reforms from the same organisms, the inability of the organisms to achieve exactly the same configuration as before means that the reformed colony will be a new consciousness with a different personality, though with remnant skills and knowledge left over from the previous configuration.
Naturally, the longer a colony exists in a particular configuration, the more skills and knowledge it will accumulate, increasing the survivability of the colony. This would lead to a state where the collective consciousnesses of the colonies would be reluctant to disband voluntarily unless presented with a suitably large body of water, but where a relatively small amount of water could fool the individual organisms into disbanding. Of course, if there really wasn't enough water, the colony could reform - as a different consciousness - but this would be a case where sentience has overtaken the limitations of evolution. Yes, the colony could reform - doing so would be better than not doing so if there really isn't enough water - but the reformed colony would no longer be the same person since its neural pathways would be different.
So, if the Wicked Witch of the West was such a colonial organism, splashing it with sufficient water to trigger the instinctual disbanding of the colony in preparation for the mating season would in effect be a sentence of death for that particular colonial sentience, even if after a little while the colonial organisms could re-form the colony. After such an event, despite all the same organisms being present, it would be a different colony with a different personality and probably fewer skills.
Given that, it is not surprising that the consciousness of the "melting" Wicked Witch could - in the moments remaining to it - recognise that it would cease to exist due to the triggering of an instinctive response of its member organisms, and bewail that fact. It would have good cause, since any reformed colony would likely have fewer valuable skills, and hence be less able to survive.
In response to the OP's comment that it might be possible to re-imprint the colony's original personality onto the reformed colony, I offer this variation, which would not be too difficult to evolve:
The colonial subunits have individual identities, and after spending a considerable amount of time next to other individuals, can remember exactly which individuals to whom they were adjacent, even after dissolution and reformation of the colony. While the colony could be reformed quite quickly, it would have a random configuration with a new personality and few skills.
However, the individual subunits memories of their previous neighbours and the neural network of the colony would allow the subunits to change their positions within the colony to recreate its previous configuration and thus restore the previous personality with its more developed skills.
However, if we are using the Wizard of Oz as the template for this species, then the behaviour above is not consistent with the reactions of the Wicked Witch of the West to being splashed by a bucketful of water:
From Frank L Baum's novel, The Wonderful Wizard of Oz:
"You are a wicked creature!" cried Dorothy. "You have no right to
take my shoe from me."
"I shall keep it, just the same," said the Witch, laughing at her,
"and some day I shall get the other one from you, too."
This made Dorothy so very angry that she picked up the bucket of
water that stood near and dashed it over the Witch, wetting her from
head to foot.
Instantly the wicked woman gave a loud cry of fear; and then, as
Dorothy looked at her in wonder, the Witch began to shrink and fall
"See what you have done!" she screamed. "In a minute I shall melt
"I'm very sorry, indeed," said Dorothy, who was truly frightened to
see the Witch actually melting away like brown sugar before her very
"Didn't you know water would be the end of me?" asked the Witch, in a
wailing, despairing voice.
"Of course not," answered Dorothy; "how should I?"
"Well, in a few minutes I shall be all melted, and you will have the
castle to yourself. I have been wicked in my day, but I never thought
a little girl like you would ever be able to melt me and end my
wicked deeds. Look out--here I go!"
or from the The Wizard of Oz movie script:
MS -- Dorothy throwing water at Scarecrow -- some of it hits the Witch in
the face -- Tin Man standing at left with the Lion --
MCU -- The water hits the Witch in the face --
MS -- The Witch screams as the water hits her -- Tin Man, Lion, Dorothy
and Scarecrow look at her --
MLS -- The Lion, Tin Man, Dorothy and Scarecrow watch the Witch as she
screams and melts away -- camera shooting past Winkies in the f.g. -- the
Witch curses as she disappears, finally only her cloak and hat remain on
the floor -- her voice fades away --
Ohhh -- you cursed brat! Look what you've
done! I'm melting! Melting! Oh -- what a
world -- what a world! Who would have
thought a good little girl like you could
destroy my beautiful wickedness!? Ohhh!
Look out! Look out! I'm going. Ohhhh!
Such behaviour would be more consistent with the behaviour I originally described, in that the colonial subunits are not capable of restoring a particular configuration through the process of dissolution and reformation of the colony.
However, the novel also describes the Wicked Witch of the West as being "cunning", so it is not beyond the bounds of possibility that this reaction may have been a sham intended to lead Dorothy and company to believe that this dissolution was fatal, so that when the colony reformed - hopefully after Dorothy and company had left the area - it could restore its long-term configuration and personality and continue pursuing its goals with the added advantage that its enemies believed it to be deceased.