The number of underwater homeowners in the Washington area decreased again in the last quarter of 2012, continuing a trend that has brought relief to thousands of borrowers. In the Washington region, the percentage of homeowners who owe more on their mortgage than their homes are worth dropped to 28 percent in the fourth quarter of 2012, compared with 32 percent during the same period a year ago, according to a report by Zillow, which tracks real estate data.