That depends entirely on how much water you dilute the cranberry juice with. If the mixture is mostly water, then yes it will be less effective unless you drink numerous diluted glasses to get a full serving of one glass of juice into your system. I doubt it would do much harm if you just add a bit of water though. However, cranberry juice is more of a preventative measure than it is a treatment. A UTI is an infection and cranberry juice alone will not treat one. It may ease the symptoms or mask the infection, but one should always consult a Doctor when a UTI is suspected. Left untreated they can be serious. I had a mild UTI once and was given a sulpha based antibiotic to clear it up. Turns out I was allergic to sulpha drugs, so I called the Dr. and he asked if I still had the UTI symptoms and I said that I didn't, so he just recommended that I discontinue the treatment and wait for the rash the sulpha drugs caused to clear up. 2 days later I was running a temperature of 102 F and had developed a severe pain in my lower back and could barely breathe. I was rushed to the hospital and spent 5 days. The infection was worse than thought and it had begun to spread to my kidneys plus I was dehydrated from the fever. UTIs are common and are easily treated. My case was extremely rare. See a Dr. if you suspect you have an infection. Hope this helps.