Combine in a stainless steel pot the water and sugar. Zest 4 of the lemons into the water and bring to a boil. Reduce to a simmer for 5 minutes. Turn off heat and strain the lemon zest out. Add to the ice to chill.
While water is coming to a boil, squeeze the juice from all 8 lemons in a blender. Add in basil and watermelon. Puree until smooth.
Once the lemon syrup is cold, combine with the basil-watermelon puree. Stir well and let sit in the cooler for 1 hour before drinking to allow time for the flavors to meld.