01 - Rub the rim of each serving glass with a lime wedge, then dip into coarse salt or sugar if desired. Set aside until ready to fill.
02 - Add watermelon cubes, lime juice, orange juice, and agave syrup to a blender. Process on high speed until completely smooth and no chunks remain.
03 - Pour the blended mixture through a fine-mesh strainer into a pitcher, using a spoon to press down on the pulp and extract as much liquid as possible. Discard the pulp.
04 - Fill the prepared glasses generously with ice cubes. Divide the strained watermelon mixture evenly between the glasses.
05 - Add fresh lime wedges and watermelon slices to each glass. Serve immediately while cold.