Dam-building is extremely costly and can create widespread environmental problems — the Eastport project, for example, would have bottled up two bays, forever altering the region’s ecosystem. Mr. Sauer and others say that placing turbines directly in the current is potentially much cheaper and more environmentally sound.

But no one knows for sure, because the underwater approach is in its infancy. While there are a few tidal current projects overseas, including one in Northern Ireland, there are only small development projects in the United States, including one undertaken by Verdant Power in New York’s East River, which also has a strong tidal current. Technologies are still being tested, and environmental questions are as yet unanswered. A tidal plant in Manhattan, Maine or elsewhere in the United States that would feed significant power to the grid is at least a few years away.

In many ways, tidal power is at a stage similar to wind power’s two or three decades ago. “That’s exactly the way wind started out, with fairly small projects,” said Robert Thresher, a research fellow with the National Renewable Energy Laboratory in Colorado, who, after years of wind-power research, now studies what is called marine hydrokinetics, a catch-all term for tidal, wave and ocean thermal energy. “They learned how to operate and maintain their machines. It was somewhat trial and error.”

Now large, efficient wind turbines are arrayed in vast farms. With tidal power, Mr. Thresher said, “I think you’ll see exactly the same kind of evolution” of the technology.

But it might not take as long, said Tim Ramsey, a project officer with the Energy Department, which began putting resources into tidal projects only two years ago.

“I don’t think we’re 20 or 30 years from being where wind is today,” Mr. Ramsey said. “It may only take us or 10 years to catch up.” Computers and other research tools are far better than they used to be, he said, and more accurate software models simulate turbine performance and efficiency.

“Our expectation is there is enough potential there to make it not only feasible but economical,” he said.