One research center here in Newcastle is putting marine devices to the test in a wave pool, and another is deploying them in the roiling ocean off the Orkneys, the low islands off northernmost Scotland. The Scottish government has pledged to generate 18 percent of its energy from renewable resources by 2010.

If marine energy replaces the burning of some fossil fuels like coal, it can help reduce overall carbon dioxide emissions and possibly increase the diversity and security of energy supply, said John Spurgeon, a marine energy specialist in the British Department of Trade and Industry. Since 1999, the government has committed more than $47 million to research and development, $93 million to commercialize that research and additional money to bring the energy into the electrical grid, Mr. Spurgeon said.

No energy source is perfect, though, and marine energy developers are running into some hurdles. While such generators do not emit smoky pollutants or leave behind radioactive waste, the machines are not small or delicate, and can be an eyesore. To draw energy from the ocean, they often need to be rooted on sea floors relatively close to shore, or mounted on rocks on the shore — places that have not traditionally been used for energy generation.

And despite their green-friendly intentions, inventors are finding some of the stiffest resistance is coming from environmental groups.

Take the case of Verdant Power, Mr. Corren’s company, which has been trying for years to erect a small field of tidal turbines in the East River — a project that may finally get started this fall. Mr. Corren, the company’s technology director, first developed the turbines as part of a New York University project in the 1980’s and planned to attach them to the Roosevelt Island Bridge.

After the school pulled the plug on the project, the design team spent years trying to find a new home. One executive even brought a prototype to Pakistan, but the data it collected was lost when the computers and instruments went missing.

Verdant embarked on a new East River turbine project in 2003, but it has taken two and a half years to get regulatory approval for the project from environmental agencies and the United States Army Corp of Engineers. The issue was not blocking the river to boat traffic, or how it would hook up to the electrical grid or even how it might mar the view, because it is mostly underwater. It was the fish population of the East River.