Pour the orange juice and measured water into a saucepan, add the sugar and heat gently until the sugar has dissolved, then simmer for 5 minutes until syrupy. Take off the heat and mix in the lemon juice.
Cut a slice off the top and bottom of each grapefruit, then cut away the rest of the peel in downward slices using a small serrated knife. Holding the fruit over a bowl, cut between the membranes to release the segments. Cut a slice off the top and bottom of the oranges, then cut away the rest of the peel. Cut into segments and add to the bowl.
Cut the melon in half, scoop out the seeds, then cut away the peel and dice the flesh. Add to the citrus fruit, then pour over the syrup. Flex the pomegranate to release the seeds, sprinkle over the salad, then chill until ready to serve.