Rashi: This is like it says "b'Sumi Anan Levusho" (Iyov 38:9; the clouds are like a garment around the ocean, which surrounds the earth - Rashi there). Tehom is the sea.

Radak #1: It discusses how the land was covered with water at the beginning of the creation. The nature of water is to be on the ground all around it. Hashem with His ability and Chachmah gathered the water to one side - "Kones ka'Ned Mei ha'Yam" (33:7), and revealed the face of the ground, for the sake of vegetation and living beings. Initially, before the water gathered, it covered the land. The suffix Vov in "Kisiso" refers to the Eretz; sometimes it is masculine - "v'Lo Nasa Osam ha'Aretz" (Bereishis 13:6). Also Tehom can be masculine or feminine - "Tehom Rabah" (ibid., 7:11), "Tehom El Tehom Korei" (above, 42:8). The prefix Beis in front of Tehom is omitted ¹ .

Radak #2: Perhaps the suffix Vov in "Kisiso" refers to the Tehom. You made the Tehom a covering on the land. This is after the water gathered. When the water gathered, You made the Tehom a cover on the land, like clothing covers a person totally, except for the face. So the water, after it gathered, was on all sides of the land, except for Pnei ha'Aretz, where dry land was seen.

Malbim: Tehom is the land under the water - "v'Choshech Al Pnei Sehom" (Bereishis 1:2). Initially it was totally covered with water, like a garment that covers [a person all] around.

Rashi: The water of the Okainus, which is higher than the entire world, stands on mountains - "ha'Korei l'Mei ha'Yam va'Yishpechem Al Pnei Kol ha'Aretz" (Amos 5:8) - spilling is always from above to below.

Radak #1: Initially, before the water gathered, it covered the entire land, valleys and even mountains, like a garment covers a person.

Radak #2: Even after the water gathered, where it was gathered it covered the mountains.

Malbim: The water surrounded the entire earth [and covered even the mountains].