I agree with what many other people have said, when I masturbate or have sex with a full bladder, my erection is harder, the feelings are incredible, my orgasm is more intense, my ejaculation is more forceful (i.e., I shoot farther), and I produce a greater quantity of semen. Normally when I cum I'm a dribbler, but if I cum with a full bladder, I'm more likely to shoot a couple of feet.
On the other hand, my ability to delay my orgasm is reduced, because the sensations are so strong I can't hold back. If I stop to pee before having an orgasm, which I often do, I lose my erection a bit and the sensations aren't as good when I get back to it.
However, I don't think it's a good idea to hold off on peeing after drinking several pints. A few times in my life I haven't been able to get to a restroom my kidneys have started to hurt. I assume that's a bad sign. When I get to that point, I go pee wherever I can, even if I have to do it where others might see (e.g., on the side of the highway).
Whether I'm having sex or I'm masturbating, I like to have a glass of water nearby (and I always offer something to drink to my partner). With a partner I'm too busy to reach for the water glass during the act; it's there because we're often thirsty afterwards. But when I'm alone I'll sip continually from the glass. So when I start masturbating, my bladder may not be full, but it gradually fills during the session. If I choose to delay my orgasm and have a long session, I'll empty and refill several times. Eventually, however, it feels so good I just can't hold off, and next thing you know I'm cleaning off the computer...