Cut the watermelon into smaller chunks and place them into a blender. Blend on high until the mixture is smooth and frothy, about 30-45 seconds, showing a bright pink, slightly thickened liquid.
Pour the blended watermelon through a strainer into a cocktail shaker to remove any pulp or fibrous bits. Use a spoon to press and extract the juice, resulting in a clear, fragrant watermelon liquid.
Add the freshly squeezed lime juice and tequila to the watermelon juice in the shaker. Add a pinch of sea salt and a handful of ice cubes. Secure the lid tightly.
Shake vigorously for about 15-20 seconds until the mixture is well chilled and slightly frothy, with condensation forming on the outside of the shaker and a scent of fresh watermelon and lime.
Strain the cocktail into chilled glasses, optionally rimming the edges with sea salt. Garnish with a small wedge of lime or a watermelon slice if desired. Serve immediately, showing the vibrant pink hue with a lightly frosted surface.
