Was it just for Fun?
What most do not seem to know is that electronics can work exposed under pure H2O since water does not conduct electricity if for example if its distilled. Regular tap water does conduct since it is the minerals that help it conduct which causes the shorting out. Most things around us is not pure which people neglect to take into factor.
I would not recommend putting trying it due to rust or how metal particles will leach into the pure water since oxygen likes to bind to other things by extracting minerals. Which is why people should not drink pure distilled water. Since it will extract minerals from your body that you need.
I do not know if in the video the guy used tap or distilled, but it did not matter as much due to being DC and not circuits. I do not know why it went black exactly or burned out but i am just guessing which I could be wrong that something got burned out to make it black from friction or maybe something broke off and gridded the inside. I most likely assume he put too much power which heated it up too much to burn it.