Beat egg yolks until thick and lemon colored in top of a double boiler. Add 1/2 cup sugar, lime juice, water, and salt; place over boiling water. Cook, stirring constantly, until thickened and smooth. Cool.
Beat egg whites (at room temperature) until foamy. Gradually add remaining 1/2 cup sugar, beating until stiff peaks form. Fold whites into lime mixture. Pour mixture into graham cracker piecrust. Bake at 325° for 10 minutes or until lightly browned. Chill at least 1 hour. Garnish with whipped cream and lime slices, if desired.