In a blender, combine watermelon cubes and cold water. Blend until completely smooth.

Add fresh basil leaves and fresh lime juice to the blender, then pulse a few times until combined.

Through a fine mesh strainer, pour the mixture into a pitcher or large bowl to remove pulp and basil remnants.

Fill glasses with ice cubes, then pour the strained cooler over the ice until the glasses are full.

Garnish with additional basil leaves or a wedge of lime if desired, and serve immediately.