Spread the chopped watermelon in an even layer on a parchment-lined baking sheet. Place the tray in the freezer and freeze until the pieces are completely solid—this will take at least 2 hours and can be done up to 12 hours in advance.

Cut the banana in half crosswise and peel one half. Reserve the other half for another use.

In a blender, combine the frozen watermelon chunks, peeled banana half, strawberries, coconut-milk yogurt, agave syrup (or honey), and salt. Blend on high speed until the mixture is silky and smooth, about 1 minute. Pause as needed to scrape down the sides and ensure even blending.

Pour the creamy smoothie into two glasses and, if desired, garnish each with a fresh watermelon wedge before serving. Enjoy immediately while cold and refreshing.