As to the cause of death of the Pharaoh of the Exodus as stated in the Qur’an, it is best that the Qur’an itself explain:
017.101-103
To Moses We did give Nine Clear Signs: As the Children of Israel: when he came to them, Pharaoh said to him: "O Moses! I consider thee, indeed, to have been worked upon by sorcery!
Moses said, “Thou knowest well that these things have been sent down by none but the Lord of the heavens and the earth as eye-opening evidence: and I consider thee indeed, O Pharaoh, to be one doomed to destruction!”
So he resolved to remove them from the face of the earth: but We did drown him and all who were with him.
and also:
043.051-56
And Pharaoh proclaimed among his people, saying: "O my people! Does not the dominion of Egypt belong to me, (witness) these streams flowing underneath my (palace)? What! see ye not then?
"Am I not better than this (Moses), who is a contemptible wretch and can scarcely express himself clearly?
“Then why are not gold bracelets bestowed on him, or (why) come (not) with him angels accompanying him in procession?”
Thus did he make fools of his people, and they obeyed him: truly were they a people rebellious (against Allah).
When at length they provoked Us, We exacted retribution from them, and We drowned them all.
And We made them (a people) of the Past and an Example to later ages.