What would be a divine image of a form that is in the water below the earth? And God spoke all these words, saying, &quot;I am Yahweh, your God, who brought you out from the land of Egypt, from the house of slaves.  There shall be for you no other gods before me. You shall not make for yourself a divine image with any form that is in the heavens above or that is in the earth below or that is in the water below the earth. You will not bow down to them, and you will not serve them, because I am Yahweh your God, a jealous God, punishing the guilt of the parents on the children on the third and on the fourth generations of those hating me, and showing loyal love to thousands of generations of those loving me and of those keeping my commandments. &quot;
Exodus 20:1-6 - LEB

The famous commandment against idolatry. I've searched and I see that the commandment against picturing forms above the heavens is directed at stopping the worship of other gods, and the one against picturing forms in the earth below would be directed at stopping worshiping the death. What about the &quot;forms that are in the water below the earth&quot;?
At first, Leviathan came to my mind, but that sea monster was in the seas, not in the water below the earth. Do we know of any tradition, either Israelite or of one of their neighbor nations, that had inhabitants in the water below the earth?
The translation "divine image" in the Hebrew is simply פֶּ֫סֶל (pesel), meaning image or idol.  The commandment in Ex 20:4 is a prohibition on making an image or likeness of anything above us, around us or below us in order to worship it.

This prohibition  against worshiping a man-made image/idol extends to everything.  The translation in the LEB, "divine image" is quite infelicitous and so misleading as it implies that the prohibition is only against images of the divine.  The word "divine" is not in the original.

The thrust of the sentence is against making an image of anything in order to worship it.  Ellicott summarises this well:


  (4) Thou shalt not make unto thee any graven image.--The two main
  clauses of the second commandment are to be read together, so as to
  form one sentence: "Thou shalt not make to thee any graven image, &amp;c.,
  so as to worship it." … It was not until the days of Hebrew decline
  and degeneracy that a narrow literalism pressed the words into an
  absolute prohibition of the arts of painting and sculpture (Philo, De
  Oraculis, ? 29). Moses himself sanctioned the cherubic forms above the
  mercy-seat, the brazen serpent, and the lilies and pomegranates of the
  golden candlestick. Solomon had lions on the steps of his throne, oxen
  under his "molten sea," and palm-trees, flowers, and cherubim on the
  walls of the Temple, "within and without" (1Kings 6:29). What the
  second commandment forbade was the worship of God under a material
  form. It asserted the spirituality of Jehovah.