Rashi: The sea will be empty of water. V'Nishesu is an expression of "ha'Sheis veha'Shever" (Eichah 3:47). The entire praise of Egypt is via the Nile river, for rain does not fall there, just the Nile rises and irrigates it via man-made rivers (irrigation channels). Therefore, the Navi compares Egypt's punishment to drying of its rivers. The sea will not turn the Nile back; the Nile will descend into the sea and not rise to irrigate Egypt.

Radak: All this is a metaphor. The Nile irrigates Egypt to the point that they do not need rain, and they get many fish from it - "Zacharnu Es ha'Dagah Asher Nochal b'Egypt Chinam" (Bamidbar 11:5). If not for the Nile, Egypt would not be settled; it would be desolate of people, for rain does not fall there. Therefore, its destruction via Melech Ashur is compared to the river ceasing, and also the sea nearby, so they cannot get any sustenance via the sea. V'Nishesu is Nif'al conjugation, like "Nashesah Gevurasam" (Yirmeyah 51:30). It is the cessation of a matter and its loss.

Malbim: The Navi tells how via losing their authority, and needing to serve and pay taxes to cruel rulers, they lost also their sustenance and income, in five ways. (a) The good of Egypt is via the Nile, which annually rises and irrigates the land. Early ones held that all rivers, including the Nile, are filled via sea water ¹ , so it depicts that the sea's water ceased, and automatically, also the Nile dried. Also refer to 19:7:1:3, 19:8:2:1*, 19:9:1:3, 19:10:1:3, 19:10:2:5.

Rashi: It is the Nile.