Place kosher salt in a saucer. Prepare a glass by slicing one lime in half and rubbing the cut edge over the rim of the glass. Lightly dip it into the salt.
Juice the limes and pour the juice into a blender. Cut the rind of the juiced limes into smaller pieces and put it into a cocktail shaker. Pour the agave nectar into the shaker and use a muddler to squeeze the limes for a minute. Pour the juice through the shaker strainer into the blender. Discard the solids.
Add the watermelon to the blender and blend until smooth. Strain the juice into the shaker.
Pour tequila, Cointreau and ice cubes into shaker, cover and shake vigorously for 30 seconds. Pour through the strainer into prepared glass. Serve immediately, garnished with a watermelon and lime slice.