Put the water and sugar in a small pan and heat gently until the sugar dissolves. Remove from the heat and allow to cool slightly.

Blend the watermelon chunks until smooth then add the lime juice, cooled sugar syrup and mix well.

Pour into a container and cover with a lid and put into the freezer. Take it out roughly every 1-2 hours to scrape down the sides that have frozen and mash them in to the rest of the mixture until it has largely frozen (you probably will mix it three or so times). Leave to freeze overnight. Alternatively, use an ice cream maker.