Unlike a rainstorm, a hurricane--or superstorm, which Sandy was demoted to--scoops up salt from oceans and estuaries the way a kid scoops up sand at a beach. The "brackish" water (salty but not extremely salty) and the saltwater (about 35 parts per million salt) get mixed up with freshwater and carried inland by the storm surge. When that water is pushed to shore, it can be more damaging than the salt-free version, especially if comes into contact with electronic equipment (as was the case with Sandy).