Put the fresh watermelon cubes into the freezer for 2-3 hours or overnight.
Place the diced watermelon and sweet condensed milk in a blender or electric mixer and blend until smooth.
Once the ice cream has reached a soft-serve consistency, you can either serve it immediately or transfer it to a sealed container and freeze it for a few hours to firm up. If you choose to freeze the ice cream, make sure to remove it from the freezer and let it soften for a few minutes before serving.
Scoop the watermelon ice cream into bowls or cones, and enjoy!