Adam Clarke on Isaiah 27:1

Leviathan - The animals here mentioned seem to be the crocodile, rigid by the stiffness of the backbone, so that he cannot readily turn himself when he pursues his prey; hence the easiest way of escaping from him is by making frequent and short turnings: the serpent or dragon, flexible and winding, which coils himself up in a circular form: and the sea monster, or whale. These are used allegorically, without doubt for great potentates, enemies and persecutors of the people of God: but to specify the particular persons or states designed by the prophet under these images, is a matter of great difficulty, and comes not necessarily with in the design of these notes. R. D. Kimchi says, leviathan is a parable concerning the kings of the Gentiles: it is the largest fish in the sea, called also תנין tannin, the dragon, or rather the whale. By these names the Grecian, Turkish, and Roman empires are intended. The dragon of the sea seems to mean some nation having a strong naval force and extensive commerce. See Kimchi on the place.