"The mangroves plunged like frightened cattle, and a fine sandy dust arose from the land and hung in a stifling cloud over the sea."

This is a simile. It compares dust rising over the land to a stifling cloud. Stifling leads the reader to believe that the dust made it hard to breathe. It also could have caused life to pause. The stifling cloud over the sea would prevent sailors from navigating their destination because they wouldn't be able to see the stars orvsun clearly.

Steinbeck is using a simile here to describe the dust that is kicked up and lingers in the air. The word stifling means a sense of suffocation. The dust cloud is like a dense cloud over the sea which will not dissipate.