Blend watermelon into a puree. Pass through a fine-mesh sieve to remove the pulp.
Pour lemon juice into a jar.
Add sugar, watermelon puree and water to the lemon juice. Stir to combine.
Taste and adjust as needed. If you like it tangier, add more lemon juice. If you like it sweeter, add more sugar. If you like it lighter, add more water.
Chill until ready to serve or serve immediately over ice.
Notes
Freshly squeezed lemon juice is best for Watermelon Lemonade but you could use storebought lemon juice in a pinch. For freshly squeezed lemon juice, make sure the lemons are at room temperature then rub them to release their juices before squuezing. Be sure to get all of the juice out of the lemons.