Albert Barnes Commentary Psalms 69:2

"I sink in deep mire, where there is no standing: I am come into deep waters, where the floods overflow me." — Psalms 69:2 (ASV)

I sink in deep mire - Margin, as in Hebrew, "the mire of the depth." This would denote either mire which was itself so deep that one could not free himself from it; or, mire found in a deep place, as at the bottom of a pit. Compare the notes at Psalms 40:2. An illustration of this might be drawn from the case of Joseph, cast by his brothers into a deep pit (Genesis 37:24); or from the case of Jeremiah, thrown into a deep dungeon: And they let down Jeremiah with cords; and in the dungeon there was no water, but mire: so Jeremiah sunk in the mire. (Jeremiah 38:6).

Where there is no standing - No solid ground; nothing for the foot to rest on. I am come into deep waters. Margin, as in Hebrew, "depth of waters." That is, waters where he could not touch the bottom - an image of some peril that threatened his life.

Where the floods overflow me - The waters. They break over my head. My life is "in danger."