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).
The steak and corn can be grilled up to a day ahead and stored in the refrigerator, in separate containers; serve them cold or at room temperature. The chimichurri can also be made up to a day ahead; store it in the refrigerator, in a jar with a thin layer of olive oil on top. Shake to combine before serving, at room temperature.