What did Hashem send from above?
Rashi (27): Hashem saved me (David) when Sha'ul was about to capture him at Sela ha'Machlekos, and an angel or messenger told Sha'ul that Pelishtim invaded (and he left to fight the Pelishtim).
Rashi (Tehilim 18:17): He sent His angels to save Yisrael from the sea and from Egypt.
Malbim (Tehilim 18:17): It is as if he already sank in the water, and Hashem pulled him out.
What is the meaning of "Yamsheni mi'Mayim Rabim"?
Radak: It is a metaphor; He saved him from many afflictions. Yamsheni is like "Min ha'Mayim Mishisihu" (Shemos 2:10), an expression of drawing and removal.
Radak (Tehilim 18:16): This is a second way to save someone drowning in water (refer to 1:16:2:2) - without defeating the enemy in war. 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.
Malbim (Tehilim 18:17): Refer to 22:17:1:1. David 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.
