Is Leviathan a descendant of Tiamat?

Like all good questions in life, the answer is yes and no! While both the story of Leviathan and the defeat of Tiamat by Marduk contain many obvious parallels, they likely do not share a common origin or source and are, instead, distant cousins that reflect the broader Mesopotamian world. The existence of one water-based monster story in Babylonia in second millennium BCE does not indicate that there must be a connection to the other, famous water-based monster story in ancient Israel. This position, that all Mesopotamian and Near Eastern cultures were derivatives of Babylonian culture, is called Panbabylonianism. 

Rather than a true school of thought, Panbabylonism was a series of related movements that began in Germany around the middle of the nineteenth century with the discovery of Babylonian tablets. Immediately, scholars attempted to find similarities between ancient Babylonian texts and the Old Testament. German Assyriologist Friedrich Delitzsch argued that Babylonia was the starting point of European culture and that all Israelite traditions were directly dependent on Babylonian traditions. He pointed, particularly, to the Genesis Creation Myth and the Flood Myth and compared them to the Epic of Gilgamesh and the Ennuma Elish. In his lecture "Bible und Babel" (Bible and Babel), he connects the word Tiamat to the Hebrew word tehom, the primordial chaos from which the heavens and the earth are created (sometimes translated as "the Deep.")  He further finds "an echo of [the] contest between Marduk and Tiamat in the Apocalypse of John, where we read of a conflict between the Archangel Michael and the ‘Beast of the Abyss, the Old Servant, which is the Devil and Satan'" (Delitzsch 45).

Friedrich Delitzsch, German Assyriologist and Professional Antisemite

Unsurprisingly for the turn of the century Germany, these ideas were built on top of a sinister undercurrent of Antisemitism. This was not merely a dry, academic debate about the diffusion of Babylonian culture in the Ancient Near East, but one part of the larger social upheavals which were sweeping through Europe at the time (the aforementioned lecture was attended by Kaiser Wilhelm II himself!). Delitzsch broadly sought to discredit Semetic cultures as mere copiers and corrupters of a pure, Aryan Babylonian culture. 

In the end, Panbabylonism was thoroughly discredited and acts as a mere historical footnote in the study of Mesopotamia. Further investigation has found the Ugaritic Ba'al cycle, which describes a sea monster named Lôtān who is defeated by the storm/rain god Hadad. The current scholarly consensus is that Leviathan from the Old Testament is much more likely to be a direct continuation of Lôtān and related ancient Semitic serpent deities rather than Tiamat or anything else in the far older Babylonian mythology. 

So yes, we should be careful to make broad generalizations about the past to avoid sliding down the Bad History to Antisemitic Conspiracy Pipeline.

"The Destruction of Leviathan" by Gustav Doré 

Sources:

http://members.westnet.com.au/gary-david-thompson/page9e.html

https://www.britannica.com/topic/Leviathan-Middle-Eastern-mythology

https://theopolisinstitute.com/leithart_post/de-assyrianization/

https://www.google.com/books/edition/Babel_and_Bible/5RgYAAAAYAAJ?hl=en&gbpv=1&printsec=frontcover - Full Text of Babel und Bible