An increasingly popular idea among the owners of oceanfront properties on Long Island has been to organize into tax districts to fund erosion-protection measures, such as pumping millions of tons of sand onto narrowed beaches. For these property owners, the districts may seem attractive, but they may well delay the day of reckoning along the coasts at a time when a more flexible response to rising sea level is needed — one that would not almost guarantee that United States taxpayers would be asked to pony up to protect coastal vacation properties.