Using a small, sharp knife, score the vanilla pod lengthways through to the centre. Put it in a saucepan with the sugar and measurement water. Heat gently until the sugar has dissolved, then remove from the heat and leave to infuse for 20 minutes.
Slice the watermelon into wedges and cut away the skin. Place the flesh in a food processor or blender and whizz until smooth, or rub through a sieve.
Remove the vanilla pod from the syrup, scrape out the seeds with the tip of a knife and add to the syrup, then beat until well mixed. Discard the pod.
Strain the watermelon purée into a freezer-proof container and stir in the vanilla syrup, lemon juice and tequila. Freeze for 3–4 hours until it is turning mushy. Mash with a fork and refreeze for 2–3 hours until it reaches the mushy stage again. Repeat the process once or twice more until the granita is evenly mushy. Freeze until required.
When ready to serve, fork through the granita to break up the ice and pile into 6 tall glasses. Serve with long spoons.