Instructions
Turn your character's vision to the right immediately after obtaining the second star coin. Look carefully at the bottom of your screen to notice the last star coin shining in the distance.
Swim toward the last star coin, which is nestled underneath a ledge. Stop once you reach a pipe in the ground. If you attempt to swim over the pipe, it will shoot an air current upward and destroy you against the rocks above.
Use your freeze attack to paralyze the urchin floating back and forth above the pipe. You must time it so you freeze the urchin when it is floating directly over the pipe. If you are successful, the urchin will fall onto the pipe and block the stream of current.
Swim over the pipe and obtain the last star coin on World 1-4.