In a large bowl, stir the water and salt together. Add the watermelon and let stand overnight.
Drain the watermelon, rinse, and drain again. In a large saucepan, combine the vinegar, sugar, cinnamon stick, ginger, and lemon juice. Bring to a boil and add the watermelon. Return to a boil and simmer for 20 minutes. Remove the cinnamon stick. Loosely pack the watermelon into jars. Return the syrup to a boil and boil for 30 minutes. Pour over the fruit in the jars and seal. Process in a boiling-water bath for 15 minutes.