In a small saucepan, bring water, lemon juice and sugar to a boil. Boil for 1-2 minutes or until sugar has dissolved. Allow to cool completely.
Using your blender, process watermelon until pureed.
Transfer the puree and sugar water to an ice cream maker. Follow the instructions that came with your ice cream maker for making sorbet.
If you don't have an ice cream maker; pour mixture into a shallow dish, place dish in freezer, stir occasionally until sorbet reaches desired consistency. If sorbet over-freezes you can allow it to thaw slightly then process it in a blender to get the desired consistency.