US President Barack Obama has marked Earth Day by announcing a plan to permit offshore electricity generation from wind, waves and ocean currents.   The move will help to reduce carbon emissions and fight global warming, while creating jobs, said Obama.  He also said that wind could create as much as 20 percent of US electricity demand by 2030, and insisted that his plan for offshore energy would not damage the economy.  "As with so many clean energy investments, it's win-win," he said, predicting that offshore energy would create as many as 250,000 jobs.