ABOARD THE SOUNDGUARDIAN, Lake Washington — The region’s cold, watery heart is nestled between Seattle and the Eastside. It uniquely supports two major roadways atop floating bridges, has offered beachgoers a summertime respite for decades and is central to the identity of the Seattle area’s culture. But Lake Washington is changing — by over half a degree Fahrenheit each recent decade. In fact, since 1963, the lake’s surface from June to September has warmed about 4.3 degrees, according to data collected and analyzed by King County and the University of Washington. While some of the lake’s war…