Nearly half of all Tampa Bay homeowners owe more on their mortgages than their houses are worth, according to data released Tuesday by CoreLogic. Of all homeowners with mortgages in the bay area, 328,449 had negative equity at the end of the fourth quarter. Another 29,000 were near negative equity. Florida's upside-down mortgage rate was 47 percent, third worst in the nation after Nevada and Arizona. About 2.1 million of the state's 4.5 million residential mortgages hold negative equity. Nationally, the rate rose slightly to 23.1 percent — 11.1 million— of all residential properties with mortgages having negative equity at the end of last year, the firm said.