All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
First of all, the stupor of death and its severity is something confirmed by the text of the Quran; Allaah says (what means): {And the stupor of death will come in truth; this is what you have been avoiding.} [Quran 50:19] Al-Aloosi said (while interpreting the above verse): “The stupor of death means its severity.” This type of severity is a kind of test and trial like other types of trials. This does not indicate a deficiency in the believer; rather, he may get good from this by having his sins wiped out or his degrees raised.
Ibn Hajar said: “The severity of death does not indicate a decrease in status, but for a believer, it is either an increase in his good deeds or an atonement for his sins.”
On the other hand, Allaah may relieve some believers of this severity. It is reported in a Saheeh (sound) Hadeeth that the Prophet said: “A martyr does not find from death except what one of you finds from a pinch.” [At-Tirmithi: Hasan Saheeh]
This Hadeeth indicates that the martyr is excluded from the pain of the stupor of death. Faydh Al-Qadeer reads: “The martyr is excluded from that as the pain that he finds is just like what others find from the pain of a pinch.”
As regards what is mentioned in the question about the devil bringing water to the person when he is breathing his last, there is no basis for this statement.
However, in Hilyat Al-Awliyaa', a Hadeeth of the Prophet was narrated by Waathilah whose wording is as follows: “Be present when the dying ones among you are breathing their last and prompt them to recite “Laa ilaaha illa Allaah (None has the right to be worshipped but Allaah)” as the wise among men and women gets mystified at that moment of severity and difficulty, and at that moment of difficulty, the devil is closer to the son of Aadam than ever before.” However, Shaykh Al-Albaani classified it as Dha’eef [weak] in Dha’eef Al-Jaami’ and in other books.
Nonetheless, it is likely that the devil would come to the believer when he is breathing his last in order to lure him. Al-Qurtubi quoted Abdullaah ibn Ahmad ibn Hanbal as saying: “I was present at the time of my father Ahmad's death while I was holding a piece of cloth in my hand in order to fasten it around his chin and over his head, and he fell unconscious and then he woke up and said while moving his hand: “not yet, not yet”. He did that several times; then I told him: “O father! What is happening? He said: “The devil was standing next to me biting his fingers and saying: “O Ahmad, you have escaped from me.”; and I was telling him: not yet, no, not until I die.”
He mentioned another story that a man was dying and he was told: “Say: Laa Ilaaha Illa Allaah”, and he was saying: “No, no”, and when he became conscious we told him what happened, then he said: “Two devils came to me, one on my right and one on my left: one says to me: “Die as a Jew as it is the best religion, and the other says: “Die as a Christian as it is the best religion”, and I was telling them: “No, no.”
However, at that moment Allaah keeps the believers firm with the saying of the word of truth "Laa ilaaha illa Allaah" as Allaah says (what means): {Allaah keeps firm those who believe, with the firm word, in worldly life and in the Hereafter. And Allaah sends astray the wrongdoers. And Allaah does what He wills.} [Quran 14:27]
Allaah Knows best.