Blend watermelon until smooth. Strain through fine mesh strainer, nutmilk bag or cheesecloth.
Stir 1 cup watermelon juice with 1 cup sugar in a small saucepan and simmer until the sugar dissolves. Remove and cool. Pour into glass jar or syrup dispenser.
Meanwhile, process ice until fluffy.
Place about a cup of shaved ice in a small glass or parfait cup. Pour watermelon syrup over to coat. Serve with a spoon and straw.
Notes
Watermelon syrup will keep refrigerated up to 7 days.