http://beta.quran.com/10/90-93And We took the Children of Israel across the sea, and Pharaoh and his soldiers pursued them in tyranny and enmity until,
when drowning overtook him, he said, "I believe that there is no deity except that in whom the Children of Israel believe, and I am of the Muslims." (10:90)
So today We will save you in body
that you may be to those who succeed you a sign. And indeed, many among the people, of Our signs, are heedless (10:92)
his body was drowned in sea, and for his body to be sign for future generation; his body shall remain intact and not harm by animals of the sea or bacteria decomposing body. till the period body got extracted to land.