Why does it say "Al Zos"?
Brachos 8a: One should pray for what is written after this ("l'Es Metzo"; refer to 32:6:2:1-5).
Radak: Since they saw that You accepted my prayer and bore my sin, every Chasid should pray to You if he sinned.
What is "l'Es Metzo"?
Brachos 8a #1: It is a proper wife - "Matza Ishah Matza Tov" (Mishlei 18:22).
Brachos 8a #2: It is Torah -- "Ki Motz'i Matza Chayim" (Mishlei 8:35).
Brachos 8a #3: It refers to an easy death -- "La'Maves Totza'os" (68:21).
Brachos 8a #4: It refers to burial.
Brachos 8a #5: It is having a bathroom nearby.
Rashi: When You are found to accept my prayer.
Radak: When he finds his heart free - it is complete in Teshuvah.
Malbim: (Right after David resolved to confess, a flood of rain descended.) Every Chasid should pray that it not reach him; it should cease. However, here it was the opposite; refer to 32:7:1:2.
Lev Eliyahu (Bereishis p.37): It is at a time when the plague of denial is found in the world and fells many, even a great Chasid and Yare Shamayim must fear and pray!
What is "l'Shetef Mayim Rabim Elav Lo Yagi'u"?
Rashi: He will not fall to the nations, who are like flooding water. We find that David prayed "Niflah Na v'Yad Hashem Ki Rabim Rachamav uv'Yad Adam Al Apoleh" (Shmuel II, 24:14).
Radak: If he does so (fully repents), it goes without saying that he will not die in his sin. Rather, even if many afflictions come to the world, like a massive flood that floods everything, it will not reach him, like it did not reach me.
