To ensure the watermelon doesn’t wobble, slice the rounded edge off its base, being careful not to cut into the flesh. Then carefully cut the top off using a sharp knife.
Spoon out the flesh, reserving a few thin slices for decoration, then put the watermelon flesh in a blender or food processor and blitz to a purée. Strain through a sieve and into a jug to remove the seeds.
Add the lemon or lime juice and caster sugar and stir until the sugar has dissolved. Pour in the soda water.
Tip the crushed ice into the watermelon ‘bowl’ and pour the watermelon mix over the top. Decorate with the mint and the reserved slices of watermelon. Serve with straws to drink straight from the watermelon.