Place the cubed watermelon in a blender or food processor and blend until smooth. Strain the puree through a fine mesh sieve to remove any remaining pulp or seeds, if desired.

In a large mixing bowl, whisk together the heavy cream, sweetened condensed milk, vanilla extract, and salt until fully combined.

Gently fold the watermelon puree into the cream mixture until well incorporated.

Pour the mixture into a freezer-safe container, smoothing the top with a spatula.

Cover the container tightly and freeze for at least 4-6 hours, or until firm.

Before serving, let the ice cream sit at room temperature for a few minutes to soften slightly for easier scooping.