Liquefy the watermelon and cucumber in a blender and strain through a fine mesh sieve into a pitcher. Refrigerate until ready to use.
Mix the lime zest and salt on a small plate and reserve.
When ready to serve:
Mix 1/2 cup watermelon-cucumber juice with lime juice, agave and tequila in a cocktail shaker filled with ice; shake well to chill. Rub the rim of a martini glass with a lime wedge and dip it into the lime zest mixture. Pour the margarita into the salt-rimmed martini glass and enjoy!
Leftover watermelon-cucumber juice can be chilled and used up to 3 days. Watermelon and cucumber juices are extremely good for you and super refreshing on a hot summer day. These delicious fresh juices replace the sweet and sour mix found in most restaurant margaritas and the result is an addictive, thirst quenching cocktail. A splash of fresh orange juice or a little Cointreu adds another fruity element. If you have a whole watermelon and cucumber, cut small wedges of each to garnish the drink.