In a medium saucepan over medium-high heat, combine the sugar, almond milk, coconut milk and cornstarch. Heat until bubbly and thickened. It will continue to thicken as it cools. Once cool, add the vanilla.
Refrigerate until thoroughly chilled.
Meanwhile, puree enough chilled watermelon cubes to equal 2-1/2 cups after straining,
Combine both mixtures and pour into an ice cream maker. Process for 20 minutes or per manufacturer's instructions.
It's ready to eat soft-serve at this point, but you can freeze for a few hours to make it scoop-able. An ice cream scoop dipped in warm water helps a lot here.
Garnish with wedges of watermelon and fresh mint.
Store in a covered container in the freezer.