During college I was working incredibly hard in the semester, and I went to a friend's home and used their family outdoor hot tub. While I was alone, I remember allowing the jets to get close to my pipes, and enjoying some free manual stimulation.
A few days after, I left class to be rushed to the hospital - when the pain hit it felt like my balls were literally being placed in a vice and were been crushed within an inch of my life. Worst pain I have ever experienced, ever. I was checked in for a 5 day stay for a urinary tract infection. Then the doc says "funny- this is normally a female condition." Nice one.
I will NEVER use a questionable private hot tub without knowing how recently it has been cleaned, and I will N E V E R bring any of my body openings anywhere near a spot where jets are forcing the water to run upstream. It's just not worth it.
Lesson learned. Heed my advice and steer clear.