Well, you could persuade people to stop purchasing bottles of water.
-Bottled water is incredibly overpriced
-Purified tap water is not only much cheaper, but is held to a higher standard and must meet several requirements as a federal utility. Water bottle companies have very lenient requirements and contain some pretty nasty things.
-Plastic is not all too biodegradable. I believe it takes several hundred years to decompose in the ground, and 100 years to decompose in warm, salt water like the ocean, leaving pollution and toxin in the water. So not only are people wasting their money on low-grade water that is 1200% more expensive than tap water, but they're also polluting the earth.
-You could also compare how a water purifier is only like 20 bucks, and then compare that to how much money is spent on bottled water per year
-Bottled water is contaminated with toxins that were leeched in from the thin, plastic bottle. These can lead to many health issues and cancer.
-Another good topic point would be to persuade people to purchase good quality refillable water bottles.