Cut watermelon into about 24 cubes, 1½ inches in size. Use a melon baller or spoon to scoop out center of each watermelon cube.
Rinse berries and pat dry. Finely dice strawberries and slice blackberries into fourths. Combine berries, mint, and lemon zest in large mixing bowl. In a small mixing bowl, whisk together cayenne pepper, honey, and kosher salt. Drizzle that over berry mixture, then gently mix until evenly coated. Marinate in refrigerator for 15 minutes.
Pour tajín onto plate and dip watermelon cubes in it to coat the tops. Place coated cubes on serving platter, then scoop berry mixture into each, filling to the top. Keep chilled until ready to serve.
Notes
Reserve removed watermelon flesh for cocktails or snacking.