Process half of the watermelon in a blender until smooth, about 45 seconds. Pour through a fine mesh strainer into a pitcher; discard solids. Repeat process with remaining watermelon.
Whisk together sugar, lime juice, and ginger in a small bowl until sugar dissolves. Add mixture to strained watermelon in pitcher; stir in chilled pineapple juice. Chill mixture at least 2 hours or up to 2 days.
Fill 8 highball glasses halfway with ice. Stir club soda and tequila, if using, into watermelon mixture. Pour evenly into glasses.