Radak: This and the coming verse are metaphors for the great Simchah that there will be due to the great Shalom and good that will be in the world. Regarding the opposite of Shalom, metaphors say "Shemesh v'Yare'ach Kadaru" (Yo'el 2:10), "Avlah Navlah ha'Aretz" (Yeshayah 24:4).

Gra (Divrei ha'Yamim I, 16:31, Mishlei 23:24): Simchah is only for something new; Gil is even for something old ¹ . Only Gil applies to the land, for "v'Ein Kol Chadash Tachas ha'Shamesh" (Koheles 1:9), but there is Chidush above the sun in Shamayim, so there is Simchah there.

Hagahah from the Gra's son (Aderes Eliyahu, Iyov 3:32): Simchah is over something that one sought. Gil is for something that happens to come to him.

Malbim: Simchah is constant joy. Gil is something that comes new. It is proper to have Simchah over the fixed conduct, and Gil over something unusual. Shamayim always acts according to natural orders; it always has Simchah. When reward comes to the land not according to the normal order, Gil applies. Both of them can be together. Shamayim and the Ma'arachah go according to their nature, and the land is conducted according to Hashgachah and recompense for deeds. This can be in two ways; refer to 96:11:3:2, 96:12:2:1.

Rashi: It will raise its voice in praise ¹ .

Malbim: This is one way that Shamayim can go according to nature and the land according to Hashgachah at the same time. The sea goes out and floods the world if it is liable.

Radak: It is fish (the sea is full of them).