Back to the castle behind the waterfall for this heart.  This time enter the door on the left wall.  You'll soon find yourself underwater.  Swim up and then jump out onto the walkway.  Continue on until you reach a pole and then up you go.  Stand on the top of the pole and jump up into the water.  Swim through the ring and then swim down, drop, and swim down again and out the door.

Next go through the door behind the middle tower.  You'll be in water again so swim up.  Jump up onto a ledge and then onto the bouncer.  Jump up to another bouncer, and then one after that.  when you reach the ceiling grab on to it.  Move to the center of the ceiling, drop through the ring, and then swim down and out.

Enter the last tower and get a move on - there's no air in this tower.  Go straight up and then through the gaps, and you'll eventually reach the ring.  After that, swim out as fast as you can.  You'll then find the heart at the exit to Timberdale.