Remove and discard the hard outer pods of the dried tamarind. Combine the remaining dried tamarind or the prepared pulp with the water and bring to a boil. Cook over medium heat about 30 minutes, or until the flesh is very soft, occasionally stirring and mashing with a whisk to break up the flesh and separate the seeds.
Strain and discard the solidsl. Cool to room temperature and chill. Serve in tall glasses with plenty of ice.