All-Purpose Bait are items found within The Legend of Zelda: The Wind Waker for the GameCube. They can be bought at Beedle's shop, from Knuckles or found by killing rats. After this you can store it in your Bait Bag. All-Purpose Bait has three primary purposes - first and foremost is feeding it to Merman. Merman can be found in the ocean surrounding all of the 49 distinct islands in the game. Once you find him swimming in the water, use the All-Purpose Bait and it'll lure him in. Following this he'll give his thanks by charting the island on your map.
The second purpose is placing it near rat holes. The rats will then give their thanks for the bait by opening up the shop for you instead of attacking you. Finally, it can also be used to make pigs dig in the spot of soil they're standing in.