Step 1In a blender, puree the watermelon with 4 cups of water and the agave nectar, in batches.
Step 2Strain through a coarse strainer or mesh sieve, discarding any solids. Stir in the lime juice and a generous pinch of sea salt.
Step 3Adjust the seasoning and consistency to taste: agave nectar for sweetness, lime juice for tartness, sea salt to enhance flavor, and water to thin. Serve over ice. Garnish each glass with a slice of lime, if desired. This will keep for 2 days, refrigerated.