Scoop your watermelon flesh into a blender puree until smooth, straining through a fine mesh sieve into a large jug. Note - you might have to do this in batches.Squeeze in the juice of 3 limes, and slice the remaining limes into thin rounds before stirring them into the jug.Stir in the tequila, Prosseco, mint, and ice.Top with the mint leaves and serve.