Place half of the watermelon and half of the water in a blender container. Blend until smooth.
Pour watermelon puree through a fine mesh strainer into a 2 quart pitcher. Discard solids. Repeat with remaining watermelon and water.
Add lime juice, agave nectar and salt to watermelon juice. Stir. Add extra lime juice and agave nectar to taste.
Serve chilled over ice. Garnish with mint sprig and lime wheel.