Istanbul, a historic megalopolis of 17 million residents between two seas and two continents, is not normally thought of as a seaside resort. But like New York, Beirut and a handful of other capitals, you can swim there all summer long and return home on the metro with salty skin and sand stuck to your sandals. A spike in popularity at the beaches has been attributed to an economic crisis that has seen consumer prices soar by 80 percent in one year. IMAGES AND SOUNDBITES