Cut seedless watermelon into 1-inch (2.5cm) cubes and spread in a single layer in a freezer-safe zip-top bag, removing as much air as possible. Freeze for at least 4-6 hours or overnight until completely solid.
Measure out the frozen strawberries, vanilla Greek yogurt, milk, honey, and lime juice and have them ready beside the blender.
Add the milk, yogurt, honey, lime juice, and salt to the blender first, then layer the frozen strawberries and frozen watermelon cubes on top.
Secure the lid and blend on low for 10-15 seconds, then gradually increase to high and blend for 45-60 seconds until completely smooth and thick; add 1-2 tablespoons of extra milk if needed to help it blend.
Taste the smoothie and add more honey for sweetness or an extra squeeze of lime for brightness, then blend for a few more seconds to incorporate.
Pour immediately into two tall glasses, garnish with fresh mint leaves and a watermelon slice on the rim, and serve right away.