It's hot, and we're thirsty. Aren't you?
Variation: Add a shot of vodka.
Variations: Use an equal measure of plums or currants if you can't find rhubarb; top off with prosecco instead of sparkling water.
Variations: Add a shot of gin or vodka.
Variations: Add a few spoonfuls of the watermelon puree and a splash of lemonade to a glass of bubbly.
Here are some more ideas: