Cut the watermelon in half crosswise, while cutting the edge of the skin with a long, sharp knife in a jagged shape. Cut out the flesh with a ball cutter or remove it with a knife and dice it.
Shake Campari, rum, lemon juice and ice cubes vigorously in a cocktail shaker. Strain the liquid into a hollowed out melon half and fill up with ginger ale. Fill melon balls into the fruit halves and serve the drink well chilled.
Results in about 2 glasses.