I came across this fantastic idea while working as an environmental educator in Belize, (see references below). I went on to share it with educators and students I worked with in the Philippines and Fiji Islands as I love how it actively engages students in telling the story and provides a strong visual impression of the problem. I have since updated it with Causes and Consequences cards and photographs to make the instructions clearer.
clean water 
polluted water
The story involves a fish travelling from its home through connected ecosystems and encountering different sources of water pollution in each. Substances like soil and soy sauce are added to the water to represent the pollutants and students consider the effect on the fish. At the end they discuss whether the fish will survive and whether it would be safe to eat it, making the link to how pollution also effects humans. A great follow up activity is asking the students to the to remove the pollutants from the water. Other ideas could be exploring how to prevent pollution at source or creating a story map.
If you would like a free download of the lesson plan including instructions, cards and editable scripts go to one of the Teachers Telling Tales online stores:
References
This activity is adapted from ‘Fishy Tales’ lesson in the Mesoamerican Barrier Reef Systems (MBRS) Project guide for primary schools. They cite their source as: Utah Lesson Plans, adapted from Chilton-Stringham, P., Wolanin, J., (1995). Water, Stones, and Fossil Bones. “Fred the Fish.”
Vocabulary
To dive deeper into the vocabulary linked to water pollution, take a look at an example of a quiz on Quizlet