Cut the watermelon into cubes, removing the rind and any seeds if not using a seedless watermelon. Spread the watermelon cubes in a single layer on a baking sheet lined with parchment paper. Freeze the watermelon cubes for at least 4 hours, or until completely frozen.
Once the watermelon cubes are frozen, place them in a high-speed blender or food processor. Add the lime juice and agave syrup (or maple syrup).
Blend until smooth, stopping to scrape down the sides as needed. This might take a few minutes, and you may need to blend in batches depending on the size of your blender or food processor.
Taste the mixture and adjust the sweetness if needed by adding more agave syrup or maple syrup. If the mixture is too thick, you can add a little bit of water, one tablespoon at a time, to help with blending.
Transfer the blended mixture to an airtight container. Freeze for at least 2 hours, or until the sorbet is firm enough to scoop.
Scoop the sorbet into bowls or cones. Garnish with fresh mint leaves if desired. Enjoy your refreshing vegan watermelon sorbet!