Place watermelon cubes in single layer on tray or in a shallow baking pan; freeze for 2 to 3 hours or until firm. Transfer to freezer bag or container until needed.
With a large spoon, remove flesh from cantaloupe, leaving 1/4-inch shell; set fruit aside. Cut thin slice from bottom of each shell so shell sits flat. Place shells, upside down, on a paper towel-lined tray; set aside.
Place cantaloupe fruit in food processor or blender; cover and blend or process until smooth. Place fruit mixture in a fine mesh sieve set over bowl. Let stand 5 minutes to remove excess liquid; discard liquid. You should have about 1 cup pulp.
In a large bowl gently fold pulp into softened frozen yogurt until just combined. Divide the frozen yogurt mixture evenly between cantaloupe shells. Place shells on a baking sheet or tray. Cover and freeze overnight until very firm.
Before serving, let shells stand at room temperature for 20 to 30 minutes to soften slightly. Top with watermelon cubes; sprinkle watermelon lightly with sea salt, if desired. Scoop into serving bowls. Makes 6 to 8 servings.