The Enuma Elish, the myth that you are referring to Tiamat and Marduk, was written in the later part of Old Babylonian (literary) period, and written in 16th century BC. Before that time, there was no evidence of this being known in the early part of Old Babylonian period.
And there is certainly no myth of Tiamat and Marduk in the older Sumerian literature. He is mention in Sumerian literature, but Marduk was a very minor god in Babylon, until the 16th century BC. There was no war in heaven or vying for powers in Sumerian legend.
There are several different versions of Sumerian myths where a god, sometimes Enlil and sometimes Enki, had created humans from clay or earth. There is even one in which humans were created by the goddess. And there were no war in any of them.
There are Sumerian versions and Akkadian-Babylonian versions in regarding to building of the ark-like boat, during the Deluge. When was Genesis composed? Centuries later, after these myths were composed, and you think that Hebrew created their own Creation and Flood myths? I don't think so, especially considering that stories of Gilgamesh and Ziusudra/Atrashasis/Uta-naphistim were known in as far west as Hittite Empire, Canaan, Syria and even in Egypt, from the evidences that clay tablets were found in all these regions. Many of these tablets may have been fragmented, but enough is recognisable that epic of Gilgamesh is recognisable and known in the region where Israel would later occupied by Joshua.
So spare me the excuse that the Hebrew came to their creation myths alone, without any influence by Sumerian/Babylonian/Egyptian.
Even the myth of Jesus in regarding to his resurrection, is not only derived from Mithra, but also from Egyptian myth about Osiris, Isis and Horus, long before any Christians wrote gospels and letters. There are also sins and cleansing of sin, self-sacrifice, resurrection and afterlife in heaven in Greece and Asia Minor, also centuries before the Christian Era.
