Add watermelon, water, lime juice, and honey to the pitcher of a blender (divide ingredients between two batches if they won't fit in one).
Puree until smooth. Pour through a fine mesh sieve or strainer into a pitcher, pressing down on the watermelon pulp with the back of a spoon to push all of the liquid through.
Refrigerate for an hour or more until chilled.
Serve as is or over ice. Stir in additional cold water or seltzer if you prefer your agua fresca more watered down