Watermelon Cooler

Prepare this drink in advance and store it in the refrigerator for up to three days. Garnish with mint and lime slices. If desired, add 1 1/2 ounces of vodka or white rum per serving.
Put half of the watermelon in a blender; whisk until smooth. Strain the watermelon through a sieve into a jug; remove the dry parts. Repeat the procedure with the remaining watermelon. Add lime juice, sugar, 1/2 cup water and a pinch of salt. Serve with ice.