As part of the Crossing the Shallows questline in Moonglow Bay, Reese will ask you to bring him a Jungle King fish in exchange for a small boat. This quest opens after completing the Tempest Eternal quest, the main story questline in which you save a fish from Lonely Rock amidst a lightning storm. After helping the sea creature, you’ll come across a door that’s too small for the Two Cats ship to pass through. That’s where Reese’s request comes in.
According to the journal, the Jungle King is a large species native to the Perching Isles. Because you won’t have access to the meat cleaver by this point, your only means of acquiring the necessary high-grade bait is exchanging low-grade bait at the Sea Market at a rate of five low-grade for one high-grade bait. To get a bunch of low-grade bait quickly, use the net fishing spot on the beach near the protagonist’s home. The bugs sometimes respawn before you even finish snatching all of them.
Use this as your low-grade farming spot until you have enough to exchange for at least 20 high-grade bait to be on the safe side. For perspective, one bug nets you three low-grade bait. Any fish can also be exchanged for low-grade bait, but we recommend against this to save them for requests or cooking recipes. Once at Perching Isles, equip the Spinner lure and high-grade bait and keep fishing until the Jungle King bites.