This idea is a little rough, so please be kind, haha.
For World Water Day 2020, I am imagining having a costume contest. In each participating city, preferably near a public waterfront of some kind, people come dressed as their favorite water-borne pathogen. The contest could have several categories, such as "best overall costume", "best protozoa", etc. I would imagine this would be a good but not too serious way to educate people as to just how many water-borne diseases are out there and, should there be supplemental materials to explain each organism, just how large of an impact they have on the world.
Perhaps as a sub-event there could even be something like a masked-wrestling match, where the competitors are dressed in some way like their respective organisms. The advantage of this is it would make describing their impact less in your face so to speak. By this I mean that in the preamble to the match where the announcer tries to hype up the crowd by describing the competitors, he (or she) could mention how many infections worldwide or even related deaths each one is responsible for under the guise of setting them up as villains for the crowd. I hope this makes sense, but let me know if it doesn't. Thanks!