Bring water and sugar to a boil; cook 2 minutes. Add quince, and cook over medium-low heat 45 minutes or until very tender. Remove from heat. Remove quince from pan with a slotted spoon, reserving liquid. Place quince in a food processor; process until smooth.
Cook reserved cooking liquid over medium-high heat, without stirring, until candy thermometer registers 234°. Remove from heat; stir in pureed quince and juice. Cool; pour into an airtight container.
Note: Refrigerate in an airtight container up to two months.