You’d want a flammable liquid so that it floats on the water and spreads more the wetter it is.

Here’s some reading on why https://www.nfpa.org/news-blogs-and-articles/blogs/2024/08/12/what-is-a-class-k-fire-extinguisher

Good luck with your story.

It only takes about a tenth grade level of understanding physics and chemistry to make a decent device from scratch. With the access Americans have to over the counter explosives (tannerite) and arduinos you only need enough of an education to know how to copy and paste code. Relying on people being uneducated isn’t a good strategy because these things are extremely easy to make. I work with some people that I question whether or not they can read who build effective devices with little to no effort.

And how do you think unions come about? Historically it’s by setting things on fire.