Put the water, sugar and lemon rind in a saucepan and cook over low heat, stirring, until the sugar dissolves.
Increase the heat to high and boil the syrup, without stirring, until the temperature reaches 220°F (104°C) on a candy thermometer or for 6 minutes, if you do not have a thermometer.
Remove the pan from the heat and add the lemon juice.
Cool the mixture and then chill it.
Strain the mixture into an ice cube tray and freeze until it becomes mushy in the middle and hard around the edges.
Turn the mixture into a bowl and beat until it is smooth.
Beat in the egg white, return the mixture to the freezer and freeze for 30 minutes.
Beat again and then freeze for 1 to 1 1/2 hours or until the water ice is frozen through.