What? You are paying his uncle's mortgage and no one is paying for the water?
Because the water is in the Uncle's name, HE, as in the UNCLE, must call and state that the house is being rented by XYZ (your boyfriend) and to put the bill in XYZ's name. In the meanwhile, because you and the boyfriend did not have an agreement with the Uncle about who pays the water, and you consumed all of that water, he may ask you to pay that water bill, so be prepared to do so.
Now, about water consumption. You can really decrease your bill by changing just a few habits:
Take shorter showers
Only do full loads of laundry
Do not leave the water running while you are:
Brushing your teeth
Or leave it running in the kitchen just to go down the drain.
If you do dishes by hand, have a dish pan full of soapy water to conserve water while cleaning and then
rinse all clean dishes at once.
Even if the water bill is changed into your bf's name, the old bill may have to be paid simultaneously.