Do prisoners really get hosed down with a firehose? And if so, why?

In many parts of the world it is considered a less than lethal form of crowd control. If you watch video of riots the big trucks spraying the crowd are basically nothing more than fire trucks.

Water is much cheaper than pepper spray or tear gas, and under pressure it can HURT.
In some prisons it is used to intimidate and quell disturbances. When you are getting hit with water from a 2” hose at 200+ psi you are going to move away or go down.

I know in Japan, prisoners and even those who are arrested and not formally convicted are sometimes hosed down with a strong jet of water, then shoved into huge refrigerators with near freezing temperatures for lengths at a time. It is common practice to make delinquents suffer if they do not co-operate with the police. Maybe that’s why the crime rate is rather low there. The consequences are so terrible, no one dares breaks the law. Just like the practice of “caning” in Singapore, for mere offenses like vandalism. And these practices usually have the full support of the public too. Yikes!

I know of a story in Germany where One Man was sprayed by a fire truck. The beam ripped right through his umbrella and in his face.
Ripped one of his eyes out, too.

It is one of the many tools that can be used to help control people.