Charles Ellicott Commentary Job 41:31

"He maketh the deep to boil like a pot: He maketh the sea like a pot of ointment." — Job 41:31 (ASV)

The sea — i.e., not necessarily the salt water, for the Nile is still called the sea by the Arabs, and so it is with many other large rivers. For example, the "sea-wall" of the Thames below Gravesend.