Add the watermelon cubes to a blender and blend on high speed for 30-45 seconds until completely smooth and liquified.
Pour the watermelon juice through a fine-mesh sieve set over a large bowl, pressing gently with a spoon to extract all the liquid. Discard the pulp.
Measure out 1 1/2 cups of strained watermelon juice and pour it into a mixing bowl. Add the coconut milk, chia seeds, maple syrup if using, vanilla extract, and a pinch of salt.
Whisk vigorously for 1-2 minutes until the chia seeds are evenly distributed and no clumps remain. The mixture will look thin at first.
Cover the bowl and refrigerate for 10 minutes, then remove and stir again to prevent the chia seeds from settling at the bottom.
Return to the refrigerator and chill for at least 4 hours or overnight, until the pudding has thickened to a spoonable, creamy consistency.
Give the pudding a final stir before serving. Divide into individual jars or bowls and top with fresh watermelon cubes, mint leaves, and toasted coconut flakes.
