Add base. Add the frozen watermelon to a blender and pour in about half of the milk.
Blend. Blend until smooth, adding more milk gradually to reach your desired thickness.
Sweeten. Add the honey or sweetener and blend again until fully combined.
Serve. Divide the milkshake between two glasses and serve immediately. Top with whipped cream if desired.
Notes
Consistency. Add more milk for a thinner drink or less milk for a thicker milkshake.
Freezing watermelon. Cut watermelon into small chunks and freeze in a single layer on a baking sheet before transferring to a bag or container to prevent clumping.
Creamier option. Add up to 1 cup ice cream or frozen yogurt for a richer, thicker shake. Start with less and adjust to taste.