In a small saucepan add water and sugar. Bring to a simmer over medium-high heat until sugar is dissolved, stirring occasionally. Remove from the heat and allow to cool completely. This can be done up to 2 weeks in advance and stored in the fridge until ready to use.

Remove the watermelon rind with a sharp knife, then roughly cut up the fruit into chunks and place it in the bowl of a large food processor or blender.

Use a lemon juicer to squeeze the lemon juice from fresh lemons that have been sliced in half. Be sure to remove any seeds from the lemons so they don't end up in your lemonade. Add the lemon juice to the cubed watermelon.

In a food processor or blender, add in your cubed watermelon and lemon juice. Blend until smooth.

Combine the simple syrup and watermelon juice mixture in a large pitcher. Add in the cold water and stir. Serve with lemon and watermelon slices!