In a small saucepan, bring water and sugar to a boil. Cook and stir until sugar is dissolved. Remove from heat; stir in lemon juice and thyme. Transfer to an 8-in. square dish; cool to room temperature.
Remove thyme sprigs. Freeze for 1 hour; stir with a fork. Freeze 2-3 hours longer or until completely frozen, stirring every 30 minutes.
Stir granita with a fork just before serving; spoon into dessert dishes. Garnish with lemon zest.