In a large pot, bring your water to a boil. Over medium heat, slowly add the berries and sugar. Boil for 10 minutes. Using a potato masher, mash the cooked berries and stir. Mix your corn starch and water together. Remove the pot from the heat and whisk in the corn starch mixture. Return to heat and stir constantly for a few minutes. Cook until thickened and clear. Cool and enjoy