From the Super Mario Wiki
When the player decides to stop here, the player will meet Sushi, a shark who just saw someone fall into the ocean. His quest, Sploosh!, is a mystery, where the player finds it was Shroomlock who went under, but managed to get back on land. He protests that someone pulled him under the water, and wants to find out who. Sushi explains that he saw three suspects that may be responsible (Blooper, Dolphin, and Cheep-Cheep), and goes through their allibies. Solving the crime will reward the player with the Gaddget, Digitizer.