Think of it as the all-American version of Venice's frosty-fruity sgroppino cocktail. Instead of the Italian version's lemons, freeze the flesh of a watermelon and puree it to an ice-cold slush. Add vodka and Prosecco to the mix (or leave them out altogether).
Don't overlook the importance of the accompaniments. Eggs, tuna, cured meats, and other protein-rich items make the otherwise light-bodied soup a satisfying meal. Set everything out in separate dishes, and let everyone pick and choose among the offerings.