Adam Clarke on Revelation 16:5

The angel of the waters - The rabbins attribute angels, not only to the four elements so called, but to almost every thing besides. We have already seen the angel of the bottomless pit, Rev 9:11, and the angel of the fire, Rev 14:18. The angel of the earth is spoken of in Yalcut Rubeni, fol. 13, 2, and is called Admael. They have also an angel that presides over the grass; another that presides over the cattle which feed upon the grass.

They say that God employed the angel of the sea to swallow up the waters at the creation, that the dry land might appear. He disobeyed, and God slew him; the name of the angel of the sea is Rahab. See Baba bathra, fol. 74, 2. It is plain from several places that the writer of the Apocalypse keeps these notions distinctly in view.