Combine the water, sugar, and salt in a saucepan. Bring to a rapid simmer to dissolve the sugar. Once the sugar is dissolved, set aside to cool.
Peel the watermelon and cut the flesh into cubes (you should have about 2 cups). Process in a blender until smooth. Strain, if desired (see note). Pour into a serving pitcher.
Juice 7 of the lemons and add to the watermelon puree. Slice the remaining lemon into thin rounds and add to the pitcher. Stir in the simple syrup. Add the seltzer and stir gently to combine. Chill well and serve over ice.
