The water heater thing doesn't make sense to me. You don't shower in water that comes from the hot water heater alone. Even at lower than 175 degrees you would be scalded. You shower in a mixture of hot water from the heater and cold water. If this bacteria is in our water supply we are going to be exposed in the shower no matter what the temperature.
Could it need a warmer environment to grow and that's why it doesn't grow in cold water?