The December Special Town Meeting approved a request from the town’s water commissioners to reduce water-usage rates by about 10 percent. The change could save the average customer “probably $50 to $100 a year,” said commission chairman Peter DeCaprio. “I don’t know if lower user rates will generate more water use, so I can’t say exactly what it will do to bills,’’ he said. “But I do know it will mean using water in Cohasset will cost less.” He said the decrease was possible because the town-owned utility saved almost $500,000 a year in a new contract with a private operator. The commission expects to ask for another rate decrease in the spring. “We’re trying to operate more efficiently, and we are,” DeCaprio said.