Radak: Even though the sea is not a residence for creations of the land, You gave Chachmah to man to go in the sea in boats. He can pass from place to place and trap fish for his food - "u'Redu bi'Dgas ha'Yam" (Bereishis 1:28). Via boats, he rules over fish - "Tzipor Shamayim u'Dgei ha'Yam Over Archos Yamim" (above, 8:9).

Malbim: They go to trap fish; some go to trap Livyasan (whales).

Chulin 67b: It is a Tahor fish. "Ga'avah Afikei Maginim" (Iyov 41:7) are its scales. "Tachtav Chadudei Cheres" (ibid., 40:29) are its fins. In the future, Hashem will make a feast for Tzadikim ¹ from its flesh - "Yichru Alav Chabarim ² ", and a Sukah for them from its skin - "ha'Semalei b'Sukos Oro" (ibid., 30-31; Bava Basra 75a).

Chidushei Agados Maharal (Bava Basra 73b): It is a very deep matter regarding the reward of Tzadikim. Their perfection via receiving [from Hashem] is called eating.

Malbim: It is a whale.

Rashi, from Avodah Zarah 3b: Hashem frolics with Livyasan, during [the last] three hours of the day. It says in Iyov (40:29) "ha'Mesachek Bo ka'Tzipor."

Radak: Livyasan frolics with all creations in the sea. They are like nothing compared to it (it is incomparably bigger); it can eat from them as it desires. This is like "v'Rozenim Mischak Lo Hu l'Chol Mivtzar Yischak" (Chabakuk 1:10). Anything that a person disgraces, he jests over and mocks it - "Hashem Yischak Lo" (above, 37:13).

Malbim: Men who go to trap Livyasan (whales) have clever strategies how to trap it.