Galveston has about 50,000 people. Average consumption per person in US is 80-100 gallons per day. Average shower is 17.2 gallons.
Leak of 3MM gallons per day for 50,000 people is 60 gallons per person per day.
Maybe the residents of Galveston only use 20-40 gallons of water a day, opting for French baths and beer over showers and water. Then their average would match the national average.
An example of government incompetence and the reason private water utilities are a better option and (as an aside) usually a very good investment.