In a blender, blend the cucumbers, water, sugar, jalapeño, mint and lime juice and zest. Pour the mixture into a sieve over a 13x9 pan. Freeze for 1 hour.

Remove from the freezer and scrape and break up the more frozen pieces around the sides. Mix the frozen bits into the rest of the more liquid-y part and place back into the freezer for another hour. Repeat the scraping and mixing process and freeze for one more hour before mixing again. Keep in the freezer until ready to serve.

To serve, divide the watermelon cubes into 6 glasses and top each with a few spoonfuls of the cucumber granita and some mint leaves.