When you are in the hallway right before you reach the room of World 3's boss Naval Piranha, start to swim slowly into the room, until you are right in front of the column of falling water. You can just see the untransformed Naval Piranha, but you haven't gone far enough to trigger Kamek's appearance. There, jump straight up, and, before you fall, aim and shoot an egg at the Piranha. When the egg hits the Naval Piranha, Kamek will appear, scream "OH MY!!!" and then flee, and the Piranha will be destroyed, without you having to battle it.