Let not the waterflood overflow me - The stream; the volume of waters. The idea is that of a flood or stream rolling along, that threatened to drown him.
Neither let the deep swallow me up - The abyss; the deep waters.
And let not the pit shut her mouth upon me - In his anguish and distress he passes here from the idea of running streams, and deep waters, to that of a well, pit, or cavern - representing himself as “in” that pit, and praying that it might not be closed upon him, leaving him in darkness and in mire, from which he could not then escape. The general idea in all these expressions is the same - that of overwhelming calamities from which he prayed to be delivered.