- Arrange watermelon cubes in a single layer on a plastic wrap or parchment-lined baking sheet and freeze overnight. (Freezing in a single layer is essential; tossing the cubes into a tupperware container and freezing will result in a big block of frozen watermelon instead of individually frozen cubes, which are much easier to blend up!)
- Place the frozen watermelon, lime juice, mint leaves, and water or rum in a food processor or strong blender. Blend until completely smooth, stopping to scrape down the sides as needed to make sure everything gets blended together. The end result should be a soft sorbet texture that's not quite scoop-able.
- Spread the mixture into a loaf pan or other freezer-safe container and freeze for at least 2 hours. The sorbet will be scoop-able after 2-3 hours and firmer after a longer freeze. If you freeze the sorbet longer than 2-3 hours, let it sit at room temperature for a couple minutes to soften before serving.