Please don't do a water mark. Like I've said before this isn't your manga so you don't have any right to water mark or put your credit in any intrusive way. With a watermark you will either put it in a solid color (people will just mark over it) or you'll cover up line work (aka ruining the manga). Either way you're ruining the experience for people all to get back at one group, who are doing the exact same thing you're doing which is spreading a manga for people to read.
It's hilarious to see a guy written so unapologetically as a 'horny loner teenager'. But I mean, we were the same back in school as well. Forget getting cream all over your face, even a girl just leaning deeply over a desk was enough to get us going.