Toast the fennel seeds in a small skillet over medium heat, shaking the skillet, until the seeds are fragrant and a shade darker, about 2 minutes. Add the sugar and 3/4 cup water and bring to a simmer, stirring constantly, to dissolve the sugar, and until the mixture is reduced by about 1/2, about 3 minutes. Remove from the heat and steep 10 minutes.
In a blender or food processor and working in batches, puree the watermelon with the fennel syrup, lemon juice, and a pinch of salt, transferring the batches to a pitcher as blended. Stir well to combine. Serve the cooler in a glass over ice and garnish with a lemon wedge.