Special equipment: an immersion blender or single-beater electric hand mixer
Cut a hole slightly smaller than the top of a plastic cup on top of the watermelon and remove the skin. Using an immersion blender (or a single-beater electric hand mixer), push the blender in the hole, breaking down the flesh inside the melon until it starts to have a liquid consistency. Add the tequila and lime juice and blend to combine until you have an even, smooth consistency. Serve immediately.