1. Using a sharp knife, carefully remove the skin and white rind from the watermelon, leaving you with about 1½kg flesh. Cut this into 2cm-thick wedges, arrange them on a large platter, and refrigerate until ready to serve.
2. For the lime syrup, combine the lime zest, sugar, vanilla bean paste, and 50ml of water in a small saucepan over medium heat. Bring to a simmer and cook for five minutes, or until bubbling and slightly syrupy. Remove from heat, allow to cool for 15 minutes, then stir in the lime juice.
3. In a mortar, combine all the ingredients for the spiced salt and lightly grind until the salt is fine.
4. To assemble, drizzle half of the syrup over the watermelon. Scatter all the herbs over the watermelon, and drizzle with olive oil. Sprinkle a quarter of the spiced salt mix on top, and serve with the remaining salt and syrup in separate bowls as desired.