How do you beat the fish?
  It looks complicated, but it's really very simple. First, that big salmon will
  poke his head out of the lake and spit at you. Duck or jump. Then he'll sink
  back into the lake. Quickly, with your Frezze Wand, make a bridge on top of
  the lake. If you manage to do it from one end of the lake to the other, you'll
  succeed and make the fish surrender. Then you can grab the Ice Cube Wand, save
  Goofy and move on to the level.

Hint by Felipe Gerhard - felipe.gerhard@gmx.net