IN some ways, biological factors enhance the chances of coastal survival. Many marine creatures are tough as nails to begin with. In tenacity the blue crab rivals the cockroach. No great adjustment in human behavior would be needed for these marine populations to begin growing again. So, while other changes would come a lot harder, the essential question is whether people will give even an inch or simply continue their profligate habits and their helter-skelter march to the sea. Between 1950 and 1984, the U.S. population in coastal areas almost doubled. The Census Bureau expects that by the early 1990s, 75 percent of the U.S. population - more than 180 million people - will reside within fifty miles of a coastline.