Nearly one in three mortgage borrowers in Florida owed more on their loans than their homes were worth in the third quarter, according to a report issued Friday by First American CoreLogic of Santa Ana, California. Fully 1.2 million of Florida’s 4.2 million mortgages — 29.2 percent — were upside down. If home prices dip another 5 percent, nearly 200,000 more Florida borrowers would slip under water. Further price declines seem a near certainty as foreclosures soar and job losses mount. Moody’s Economy.com forecasts a 10 percent home price decline in Palm Beach County, while Veros Real Estate Solutions predicts a 15 percent drop for Palm Beach County and the Treasure Coast in the coming year.