"Sally ship" was not a ship but a method of loosening a vessel that ran aground from the mud holding her fast. In the days before sophisticated navigation equipment, ships ran aground much more often than today. A grounded ship could be freed with little or no hull damage if she could be rocked out of her muddy predicament.
To free her, the order was given to "sally ship". The crew gathered in a line along one side and then ran from port to starboard and back and forth until the vessel began to roll. Often the rolling broke the mud's suction and she could be pulled free and gotten underway.
