What is "bi'Yven Metzulah"?
Rashi: It is mud of the depth.
Radak: It is an affliction amidst an affliction. A person can drown in the depth of water, and he can drown in mud even if it is not deep.
Malbim: The verse hints to three Tzaros. Yeven Metzulah is sticky mud under his feet, and it is hard to remove his feet from there. Also refer to 69:3:3:3, 69:3:4:1. This is a metaphor for the place of Yerushalayim becoming muddy via the rebellion and the many enemies, and the danger is great, and the rapid current forced David to flee the land.
Why does it say "v'Ein Ma'amad"?
Radak: Not one of my limbs is standing; all sank.
Malbim: There is nothing below on which I can stand.
Why does it say also "Basi v'Ma'amakei Mayim"?
Radak: He repeats the matter in telling his afflictions.
Radak (36:7): I entered a place of demise, from which a person is not saved.
Malbim: It is a place of deep water.
What is "Shiboles"?
Rashi: It is the strong current of the river. Radak - v'Shiboles is like Oh Shiboles. In the depths, one drowns; he is not washed away. In Shiboles ha'Nahar he is washed away, even though it is not deep. All of this is a hint to great Tzaros.
