Dwr Cymru Welsh Water also announced that finance director Chris Jones will be taking on the chief executive role when Nigel Annett steps down.
The firm, with three million customers, said it had kept bills to the same level in real terms as in 2001.
Meanwhile, research by Cardiff University claims the company is worth £1bn a year to the Welsh economy.
Its financial results come in a different form to companies with shares on the stock market.
It is owned by the not-for-profit company Glas Cymru and savings or efficiencies are ploughed back into Dwr Cymru or used to keep bills as low as possible.
Mr Annett, with Mr Jones, founded Glas Cymru 12 years ago, and he said the idea stems from the notion that having a public service company, which is a monopoly, being run on the basis of profit maximisation for shareholders does not work in the customers' interest.
He said there were cases where the Glas Cymru non-profit model of ownership could work elsewhere in other businesses and industries.
Mr Jones said of the job awaiting him: "There are some big challenges ahead.
"We're a company that has to have a long-term vision, looking to invest in assets that will be around for the next for the next 20, 30 and 40 years, so we have to make the right decisions.
"We have to balance that investment with the need to strive to keep bills down for our customers as well because everyone is feeling the pinch in difficult circumstances."
Research also published on Thursday by Cardiff Business School said that Dwr Cymru contributes £1bn into the Welsh economy every year.
The study claims for every £1 spent by the company, another 56p is generated for the rest of the Welsh economy.
The report also quotes the water regulator Ofwat saying that between 2009/10 and 2014/15 Dwr Cymru customers face the lowest increase in average household bills of all water and sewerage companies in England and Wales.
On top of the £338m spent on infrastructure last year, the company is committing another £650m over the next two years, which it is argued will employ 1,000 construction workers.
The company has also reported that their financial reserves amount to £1.6bn, that's a tenfold increase compared with 2001 when Dwr Cymru was taken over by Glas Cymru.
Dwr Cymru's borrowing or gearing has also come down. It now stands at 63% compared with 65% last year and 93% in 2001.
Welsh Water put £338m into capital projects in the past year, up £76m on 2012, its financial results show.