The bet on this technology was not an obvious one; the recent history of desalination in the United States and Australia has been mixed, at best. Some recently constructed Australian plants are flourishing while others stand idle some of the time. In this country, technological missteps, delays and bankruptcies dogged the first big plant, which finally opened in Tampa in 2007.

“Tampa was a buzz kill for the sector,” Mr. Moore said.

So the Carlsbad plant is being watched not just for its performance or its effect on the local marine environment, but for its financial architecture.

Mr. Moore and other financial advisers are trying to make investors and bondholders comfortable with the technology by mimicking the financial approach of a merchant power plant — for instance, substituting a “water purchase agreement” for a “power purchase agreement,” to show that Carlsbad’s water has a guaranteed market.

The water purchase agreement was signed by the San Diego authority and the plant’s developer, Poseidon Resources, of Stamford, Conn., in late November. Poseidon bears the responsibility for completing the plant and operating it; the authority does not pay for any water that is not delivered.

The project’s costs are financed by two bond offerings totaling $734 million and a $189 million equity investment. In addition, the water authority is committing about $80 million to other capital needs. All of these arrangements have interlocking guarantees and risks, with the costs of constructing the plant borne by the project developers and the water authority responsible for constructing a 10-mile pipeline to send the water on its way to San Diego’s taps.

The public water authority did not want its ratepayers to be responsible for paying for water that was never delivered; it will pay only for water that meets its standards and goes into its reservoirs. That said, when the water is flowing in 2016 the county must pay as much as $113 million annually, which could rise over time.