What did Hashem send from above?
Rashi #1: He sent His angels to save Yisrael from the sea and from Egypt.
Rashi #2: This refers to saving David himself. When Sha'ul was about to capture him at Sela ha'Machlekos, an angel or messenger told Sha'ul that Pelishtim invaded (and he left to fight the Pelishtim).
Malbim: It is as if he already sank in the water, and Hashem pulled him out.
What is the meaning of "Yamsheni mi'Mayim Rabim"?
Rashi: This is an expression of drawing and removal, like "Min ha'Mayim Mishisihu" (Shemos 2:10). Radak (16) - this is a second way to save someone drowning in water (refer to 1:16:2:2). And so David was saved without his enemies falling in war, e.g. at Sela ha'Machlekos. David thanked Hashem for saving him mi'Mayim Rabim - in both ways.
Radak (Shmuel II, 22:16): Mayim Rabim is a metaphor; Hashem saved David from many afflictions.
Malbim: Refer to 18:17:2:1. David's enemies saw that he sank, and has a way to escape. They stood on the bank and waited - when he wants to leave, they will submerge him again. Hashem saved him also from this - mi'Mayim Rabim.
