Line refrigerated pie crust with wax paper and fill with dried beans or pie weights; bake shell in middle of oven 10 minutes; remove paper and weights and bake shell until golden, about 12 minutes longer; cool.
Lower oven temperature to 350°F.
For filling: In a medium saucepan, whisk together sugar, cornstarch and salt and gradually whisk in water and milk, whisking until cornstarch is dissolved; in a bowl whisk together egg yolks; cook milk mixture over medium heat, whisking, until it comes to a boil; gradually whisk about 1 cup milk mixture into yolks and whisk yolk mixture back into milk mixture remaining in saucepan; simmer, whisking, 3 minutes; remove pan from heat and whisk in butter, lemon juice and zest until butter is melted; press plastic wrap directly onto surface of custard.
For meringue: Beat egg whites with cream of tartar and a pinch of salt until they hold soft peaks; beat in sugar in a slow stream, beating, until meringue just holds stiff peaks.
Pour lemon custard into shell and spread meringue on top, covering filling completely and sealing it to pastry; draw meringue up into peaks and bake pie in middle of oven until meringue is golden, about 15 minutes; cool completely before serving.