2. Make a syrup by combining the water, sugar and 2 of the lime skulls (the peel after juicing) in a saucepan and bring to a boil. Set aside to cool and remove the skulls.
3. Puree the watermelon cubes in a food processor or blender, then strain through a coarse strainer into the bowl containing the lime juice. Stir in the vanilla extract and cooled sugar syrup.
4. Pour into ice cube trays and freeze overnight. (The recipe can be made up to this point and kept frozen up to 3 days in advance.)
5. When ready to serve, turn the watermelon ice cubes into a food processor fitted with a metal blade. Pulse the machine until the granita has the texture of crushed ice. Dot with the currants, making them look like watermelon seeds. Serve immediately or keep frozen until ready to serve.