I can't figure how water has much to do with UTI, diarrhea yes. I suppose if the water has a high mineral content then that might lead to excess excretion in the urine, or if there is a bad taste to the dog they might not drink as much as they should leading to possible increase in UTIs. Just thinking out loud.
Sounds like you have things under control. I've seen many patients over the years that seem to have a propensity for developing UTIs. I wonder if it's something in their own system or a structural anomaly? I've had some that do cranberry tablets or vit C tablets to try to prevent them.