Blend watermelon until pureed. Measure out 3 cups of watermelon juice, into a bowl or large glass container. Add apple cider vinegar and maple syrup, stir for about 30 seconds to fully combine.
Cover container and move to the refrigerator, letting it sit for at least 4 hours, up to 2 days.
Then use a fine strainer to let the mixture separate, pushing down just a little on the fruit. Be gentle.
Keep the syrup and pour into container, discard the pulp.
To make a mocktail, put salt on a sheet pan or plate. Use a cut lime to trace the rim of the glass, then spin the rim in salt.
Gently put ice cubes in a glass. Pour shrub and lime juice over the top. Then finish with the sparkling water. Stir with spoon. Add lime twist for garnish.