Hi, I masturbated into my bathtub and now I'm worried that It's going to be harmful. I'm afraid that because my penis was underwater, I'm going to get any diseases that were in the water (including those on my skin from before the bath). I should mention that no one else had used the bath for over a week. I also was on a plane/ in multiple airports recently and I hadn't bathed or showered before this time.
What leads you to believe you would get a disease by masturbating underwater in the bathtub? I'm 99.9% sure you don't have anything to worry about.
People go swimming all the time without getting icky infected penis disease? Why would the bath tub be any worse? I'm sure your bath tub is cleaner than many lakes / rivers / whatever people may be swimming in.
He might also be worried about any soap or other irritants in the water possibly getting in through his urethra. I don't think it's an issue.
If anything, the bath is more of the issue than the ejaculation. With or without masturbating in the tub, if something wanted to get up your urethra, it would have. You're probably fine though. Its pretty hard for guys to get a UTI, especially via these means.