Line a large baking sheet with nonstick foil (or a silpat baking mat). You may need two pans, depending on size.
Place cubed watermelon on lined baking sheet in a single layer, separating cubes so they do not touch.
Freeze watermelon until it's frozen through, about 2 to 3 hours. (This can be done in advance.)
Once watermelon is frozen, let it sit at room temperature for about 10 minutes. Then add to blender, along with sugar, lemon juice, and ¼ cup cold water.
Blend watermelon mixture until it's smooth, thick, and slushy, adding more water only if needed.
Transfer slush to glasses, then serve with a straw and a spoon (if needed). Enjoy!
Notes
This recipe make about 40 ounces (just shy of 1.2 liters), which equals 4 larger 10-ounce (1 ¼ cup) drinks, or 5 smaller 8-ounce (1 cup) drinks.The slush should be very thick when you are done. I like serving it with a spoon and a straw. The spoon for eating right away, and the straw for sipping the slush once it melts a bit.Check out the info before this recipe card for helpful tips, recipe variations, and more!