1. Place the water in a small saucepan and add the sugar. Cook over medium heat, stirring frequently, until the mixture comes to a boil.
2. Simmer for a few minutes or until the sugar is completely dissolved. Remove the pan from the heat and allow the simple syrup to cool completely before using.
3. Meanwhile, juice the lemons into a large pitcher (I set a small sieve over the pitcher to catch any seeds). Place the watermelon cubes in a blender (you may need to do this in batches) and puree until the watermelon is completely juiced (See NOTE).
4. Add the watermelon juice to the pitcher with the lemons. Taste the mixture and add as much of the simple syrup as desired to meet your tastes. I would start with ¼ cup of the syrup and add from there, if needed. It really depends on your preference and the sweetness of the watermelon/tartness of the lemons.
5. Chill the mixture thoroughly before serving. I found that this tasted best after chilling in the fridge for several hours.
6. To serve, pour into glasses filled with ice and garnish with lemon slices and additional watermelon slices/cubes. I like mine with a splash of ginger beer or seltzer as well for some fizziness!

NOTES:

1. I have a high-powered blender and did not feel the need to strain the watermelon juice. This is an optional step you can take.
2. Additionally, if you don’t want to make the simple syrup, try sweetening this with a tablespoon or two of honey. Just make sure to taste it before you sweeten it – the watermelon may be sweet enough for your tastes!