Burst 15 fish schools in the jetstream.
You can get this as early as the second jetstream. A jetstream is a fast water current that will pull you along through certain areas of the game. While going through the jetstreams just run into the schools of fish to burst through them. You will know that you have gone through them because the game will make musical noise and the school of fish will light up after you have gone through them. There will be multiple times throughout the game where you will have to go into a jetstream to make your way through the game so you should be able to burst through 15 schools of fish by the end of the game.