Place the frozen fruit, ice cubes, rum, sweetener, and lime juice into the blender. Start blending on low to break down the larger pieces.
Increase the speed to high and blend until the mixture is smooth and icy, with a thick, slushy consistency that feels cold and slightly granular to the touch.
Stop the blender and taste the slushie; if it's too thick, add a splash of soda water and pulse briefly to loosen. Adjust sweetness if desired by adding more simple syrup or honey.
Pour the icy mixture into glasses, garnishing with fresh fruit or a sprig of mint if you like. Serve immediately with a straw or spoon for savoring every frosty sip.