Place the watermelon cubes on a cutting board and use a sharp knife to remove any small seed fragments, then transfer them to the blender.
Add the freshly juiced lime, honey or simple syrup (if using), and cold water to the blender with the watermelon.
Secure the lid on the blender and blend on high speed until the mixture is completely smooth and slightly frothy, about 30-45 seconds. The consistency should be liquid with no large fruit chunks.
Place a fine mesh strainer over a large bowl or pitcher, then pour the blended mixture through it, using a spatula or spoon to slowly press the liquid through and remove pulp and seed particles.
Pour the strained watermelon lime water into glasses filled with ice, garnish with lime slices if desired, and serve immediately while cold and refreshing.