In a saucepan, bring rhubarb and water to a boil. Reduce heat; simmer, uncovered, until rhubarb is tender, 10-15 minutes. Drain, reserving liquid; discard pulp. Measure 4 cups liquid; return to the pan.
Add sugar; bring to a boil. Combine cornstarch and cold water until smooth; stir into rhubarb liquid. Cook and stir until slightly thickened, 1-2 minutes. Pour into individual dishes. Refrigerate for at least 4 hours before serving. If desired, serve with cream.