John Wesley on Job 41:31

Boil - To swell, and foam, and froth by his strong and vehement motion, as any liquor does when it is boiled in a pot, especially boiling ointment. The sea - The great river Nile, is called a sea, both in scripture, as Isa 11:15, and in other authors, as Euphrates is called the sea of Babylon, Isa 21:1 Jer 51:36. Lakes also are most frequently called seas both in the Old and New Testament: and in such lakes the crocodiles are as well as in the Nile.