Prep the produce: Remove watermelon rind, cube the fruit, and chill it. Peel and chop cucumber.
Juice the lime.
Layer the blender: Add coconut water (or water) first, then watermelon, cucumber, lime juice, mint, and a pinch of salt.
Add ice and sweetener: Toss in ice if you want it frosty. Add honey or maple only if your watermelon isn’t very sweet.
Blend smooth: Start on low to break up the ice and mint, then blend on high for 30–45 seconds until silky.
Taste and adjust: Add more lime for brightness, a splash of water if too thick, or a bit more mint if you want it cooler.
Serve cold: Pour into chilled glasses. Garnish with a mint sprig or a thin cucumber slice if you’re feeling fancy.