From the Super Mario Wiki
“Thing is, now the captain gots so big, the BOWL gots too small for him!”
Pook is a young boy and a resident of Flipside, from Super Paper Mario. He is fascinated by sea creatures, and wants to be a sailor when he grows up. He hopes to leave Flipside when he grows up and listen to the quiet sounds of the waves at sea, every day. He used to have a pet fish he named Captain Gills that he found somewhere, and kept him in a cup. Captain Gills got too big for the cup so he searched for something bigger to put him in. Merlon generously gave him his Goldfish Bowl but not too long after Mario arrived in need of it. Pook agreed to give him the bowl, only if they would give a new home to Captain Gills. The player then must go stand near the edge of the water to release Pook's goldfish. "Now the fishbowl is empty! That means you can use it as a space helmet!" Pook wished good luck to the Captain in his new life at sea. If Mario talks to Pook before getting to Chapter 4: Outer Space, he will talk about a problem that keeps getting bigger and bigger. This "problem" Pook is speaking of may be Captain Gills, as the fish grows in size every day.