Pour the wine into ice cube trays and freeze until firm, at least 6 hours (they will still be a bit icy / slushy).
Cube watermelon and freeze on a parchment paper-lined sheet pan until firm, at least 4 hours.
When the wine and watermelon cubes are frozen, you can transfer them to a plastic bag and freeze for up to a month.
Make the simple syrup by combining sugar and water. Heat them on the stovetop or in the microwave until the sugar dissolves.
When ready to serve, transfer frozen wine and watermelon to a blender and blend until smooth. (I had to let them defrost at room temperature for a few minutes before they would blend.)
Add simple syrup a bit at a time, tasting as you go. Depending on the natural sweetness of the wine and fruit, you may just need a little or none at all.