More homeowners in the Boston area are coming up for air. A new report from the real estate firm Zillow Inc. shows 122,653 borrowers -- or 15 percent -- were “underwater” on their mortgages in the second quarter, meaning they owed more to lenders than their homes were worth. That was down from 15.9 percent in the first quarter of 2013 and 19.6 percent during the second quarter of last year.