In a medium saucepan, combine 1 cup of water, ¾ cup of caster sugar, and 1 tablespoon of fresh lemon juice. Stir over low heat until dissolved, about 2-3 minutes, then bring to a boil for 5 minutes and cool for 30 minutes.

Cut 8 cups of ripe watermelon into cubes, then blend until smooth for about 1-2 minutes.

Strain the blended watermelon puree through a fine sieve into a large bowl, discarding any solids.

Pour the cooled sugar syrup into the strained watermelon juice and stir well.

Cover and refrigerate the mixture for at least 2 hours.

Pour the chilled mixture into your ice cream machine and churn according to the instructions for about 20-30 minutes.

Transfer the churned sorbet into an airtight container and freeze for several hours or overnight.

Before serving, let the sorbet sit at room temperature for a few minutes to soften.