Cut the watermelon into roughly 1-inch cubes using the knife and cutting board, ensuring the removal of any rind or seeds.
Place the watermelon cubes into the blender jar, adding the lime juice and honey or agave syrup for sweetness. Secure the lid.
Blend on high speed until the mixture is completely smooth and the watermelon is fully broken down, about 30 seconds. Stop and scrape down the sides if needed.
Place the fine mesh strainer over a large jug or pitcher, pour the blended mixture through it, and use the back of a spoon to press out as much juice as possible, leaving behind pulp and solids.
Chill the strained Agua Fresca in the refrigerator for at least 30 minutes. When ready to serve, fill glasses with ice cubes and pour the cold beverage over the ice.
Garnish each glass with a small wedge of lime or a thin slice of watermelon if desired, then serve immediately with a straw or stirring spoon.
