A massive rainstorm that produced flash floods is proving costly for northern New England: Hundreds of cars were damaged by high water around the region, with some of them totaled and others needing expensive repairs, officials said Thursday.    AAA of Northern New England had to call in extra tow trucks Wednesday as it responded to about 1,600 calls - about 600 more than a typical day - across the three-state region.    About 250 cars and trucks were flooded in Portland, where the rainfall dislodged manhole covers and waist-high floodwaters caused cars to stall out, AAA spokesman Pat Moody said. Hundreds more were likely flooded across the region, he said.    More than 6 inches of rain fell Wednesday in Maine, while New Hampshire got more than 5 inches. Vermont also got heavy rain, but not as much.    Kathleen Rusney from the Maine Emergency Management Agency said she was surprised to see how many motorists tried to drive through floodwaters.    "We have a slogan, 'Turn around, don't drown,'" she said. "You see these people and they're just not thinking it through. It's astonishing."    There were no reports of deaths or serious injury in Maine, New Hampshire and Vermont. In Massachusetts, a 54-year-old man drowned when he jumped in the water after his boat broke away from its mooring in Swampscott Harbor, officials said.    In Maine, a National Guard helicopter on Thursday picked up three hikers stranded on the Appalachian Trail. The three, Amy McConaughy, 27, of Belair, Maryland; Kelly Wood, 25, of Nokesville, Virginia; and Gabriel Grace, 26, of Baltimore, Maryland, were unharmed, officials said.    The Maine Warden Service warned hikers to beware of high waters.    Across the region, insurance adjusters were busy Thursday taking a look at flood-ravaged cars, and tow truck drivers were still hauling damaged ones, Moody said.    In Portland, it was the sixth-rainiest day ever recorded, with 5.63 inches of rain, said Margaret Curtis, a meteorologist with the National Weather Service in Gray.    Other areas of the state got even more, with an unconfirmed report of 9.75 inches in Searsport, 7.5 inches in West Rockport, 6.9 inches in Cumberland, 6.8 inches in Windham, 6.6 inches in Lewiston and 6.3 inches in Standish.