Blend the watermelon cubes until smooth, then strain the mixture to remove pulp.

In a mixing bowl, combine the strained watermelon juice, sugar, and lemon juice. Stir until the sugar is dissolved.

In a small bowl, sprinkle gelatin over cold water and let it sit for about 5 minutes to bloom.

Heat the bloomed gelatin in the microwave for about 10 seconds until dissolved, then mix it into the watermelon mixture.

In another bowl, gently fold the whipped cream and mascarpone cheese together until smooth.

Carefully fold the creamy mixture into the watermelon mixture until well combined.

Pour the pudding into serving dishes and refrigerate for at least 2 hours until set.

Serve chilled, optionally garnished with mint leaves or additional watermelon pieces.