Pick the grapes off the stems and wash them well. Drain then place in a medium pot/saucepan and add the water and sugar.
Place the pot/pan over a medium heat and bring to a simmer, stirring now and then to dissolve the sugar. Simmer for around 5 - 8 minutes so that the skins burst and the grapes become very soft. There should be plenty of nicely purple liquid and some seeds will start to come out from the pulp.
Remove from the heat and leave to cool a couple minutes then strain to remove the seeds and skins. Place the strained mixture in the fridge to cool completely, around an hour or so.
Following your ice cream maker's instructions, pour the chilled grape mixture into the ice cream maker and churn until it reaches a good consistency. Then, transfer to a container to freeze until firmer and for longer storage. When you are ready to serve, allow it to sit a minute or two to soften slightly before scooping (unlike me who got rougher scoops as a result!)
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Notes
This is still a sweet sorbet, but is slightly lower on sugar by sorbet standards - if you like, you can increase the sugar, up to about double as written.