Complex sentence: Even by 2013, nearly half of all home mortgages in the city were "underwater", with owners owing more than their houses were worth--on average, homeowners with mortgages were indebted for about 45% more than the original value of their homes.
Split into simpler sentences: Even by 2013, nearly half of all home mortgages in the city were "underwater", with owners owing more than their houses were worth. On average, homeowners with mortgages were indebted for about 45% more than the original value of their homes.