Combine water, sugar, and orange zest in heavy medium-sized saucepan. Stir over high heat until sugar dissolves and syrup boils; remove syrup from heat.

Cut all peel and pith from oranges. Working over a bowl to catch juices, cut between membranes to release orange segments. Discard any seeds. Transfer orange mixture to processor; purée until smooth, about 30 seconds. Measure 2 1/3 cups orange purée and mix into orange syrup (reserve any remaining purée for another use). Cover orange mixture; refrigerate at least 6 hours and up to 1 day to blend flavors.

Process orange mixture in ice cream maker according to manufacturer's instructions. Transfer to container; cover and freeze. Makes about 3 cups. (Can be made 2 days ahead.)