Sounds like it is related to that. Try not to mess with it and see if it goes away!
You aren't going to get an std like herpes from the water. Unless you had unprotected sex in the tub. :>) This is just on your penis? You can get hot tub folliculitis which is not serious but also would not just be red marks on your penis. You'd itch and have it elsewhere. Remember too, hot tubs are kept clean with chemicals and those can have an effect. I'd say that is most likely the reason along with as you say, the netting on your shorts. If it doesn't go away, get it looked at but would not really worry.