The duck is in the bottom of two boxes stacked on top of each other next to the water heater. It's the one with packing peanuts in it, shake those out and the duck falls out too.

Plunger the toilet, the duck will be stuck in the plunger after a few pushes.

Feed a piece of pizza to the creature in the dark tunnel, it will spit out the duck after it burps.

To find the submarine duck look through the periscope and turn around until you see the duck, it will float towards you then fall into the sub.