Pour water in a saucepan. Bring to a boil.
Add sugar, ketchup, and vinegar. Let the mixture boil once more. Stir using a small ladle.
Add the pineapple, onion, and bell peppers. Cover and simmer for 12 to 15 minutes.
Pour-in the cornstarch diluted in water. Stir.
Once the texture gets thick, turn off the heat. Transfer the sauce to a bowl or gravy boat. You can also let it cool down and secure in ball jars. This can be refrigerated or placed in the freezer for future use.