Peel and roughly dice the watermelon, setting aside a few cubes for the garnish. Wash the cucumber, cut 1/2 cucumber into thin slices and set aside for the garnish. Peel the remaining cucumbers and cut into coarse pieces. Puree cucumbers and melon pieces in a blender, pass through a sieve and collect the juice.
Mix lime juice, vodka and sugar. Add to the fruit juice, mix and chill for at least 30 minutes. Just before serving, add cucumber slices, melon pieces and ice cubes.