International Herald Tribune: For years, technological visionaries have painted a seductive vision of using ocean tides and waves to produce power. They foresaw large installations off coastlines and in tidal estuaries that could provide as much as 10 percent of U.S. electricity needs. But the technical difficulties of making such systems work are proving formidable. Last year, a wave-power machine sank off the Oregon coast. Blades have broken off experimental tidal turbines in the turbulent East River in New …