Radak: It is water that is drawn forcefully. All places where water is drawn forcefully are called Afikim for this reason.

Rashi (from Shemos Rabah 21:6): The depths were cleaved. When Yam Suf split, all waters in the world split.

Radak (Tehilim 18:16): There are two ways to save someone drowning in water. (For the second way, Refer to 22:17:2:2.) (a) Hashem widens the water, and the Afikim are seen and the foundations of the world, which were covered with water, are revealed. This is via Hashem's rebuke of the water - "va'Yig'ar b'Yam Suf va'Yecherav" (106:9) - and from the breath of His nose, which dried the water ¹ . This is a metaphor for defeating the enemy in war, and David is saved. And so David was saved without his enemies falling in war, e.g. at Sela ha'Machlekos.

Malbim (Tehilim 18:16): Hashem shattered the clouds to cease the thunder, but this led to a new affliction - a great flood, to the point that water flowed forcefully and the foundations of the world were revealed. I.e. in the six days of Creation, water covered the entire world. Hashem's rebuke sent the water and revealed the dry land; now, His rebuke returned the water to cover the land; also David was taken by the raging flow.

Rashi: It is from the strength of the blowing of wind of His nose.