Pour tequila and lime juice in a blender, followed by watermelon and berries.
Blend until smooth.
Pour into salt or Tajin* rimmed glasses and serve (*see recipe note)
Tajin is an amazing chili + lime spice that is shockingly good for cocktails - find it at your local grocery store on Amazon - linked here. Pour a mixture of coarse kosher salt and Tajin into a shallow dish. Run a lime wedge (or sliver of watermelon) around the edge of a glass, then invert and gently press into the salt mixture.