Most San Diegans are drinking water containing hexavalent chromium at levels higher than the goals set by California’s scientists, but that fall well below the state’s legal limits for the toxic metal, a new report by the Environmental Working Group found. The report, released Wednesday, seeks to draw attention to a chemical that’s ubiquitous in drinking water in more than 200 million Americans. It also highlights the tension between maximizing public health and setting limits that regulators deem practical.