The factors to consider when making a public service announcement (PSA) are essentially the same as those for a commercial. You should have a memorable slogan (e.g., "Only You Can Prevent Forest Fires," "A Mind Is a Terrible Thing To Waste"), strong visual appeal (particularly since this is an animation), and perhaps a character to deliver the information.
Before all these, you need to establish clearly what your message is and how to back it up. The message might be something like "we all need to play our part to conserve water." A PSA of thirty seconds or so will have time for a maximum of seven points to back up this message, though five points would be a more realistic goal. One of them might be the amount of water the average American uses or wastes on a daily/monthly/yearly basis. This lends itself to strong visual representation (using an animation of the number of baths/swimming pools an individual, a family, or a city could fill).
The other four points would be actions you can take to conserve water, each one visually represented. You could have the PSA narrated by an ocean-dwelling creature such as a fish, a dolphin, or a turtle, and end with a slogan, perhaps based on the idea of saving ("Save Water, Save the Planet" or something of that kind). The character who narrates the PSA could be depicted at the beginning trying to swim in an inadequate amount of water, then at the end, swimming freely in the ocean.