Bring fruit, peels, and water to a boil in a large saucepan. Cook for 5 minutes. Turn off heat, cover, and let cool. Refrigerate for 8 hours (or up to 1 day).
Freeze a plate. Uncover citrus mixture, and bring to a simmer over medium-high heat. Cook until thickest peel is tender, about 20 minutes. Measure mixture, and return to pan. For each cup of mixture, add 3/4 cup sugar.
Bring mixture to a boil, stirring often. Cook until mixture registers 220 degrees to 222 degrees on a candy thermometer, about 20 minutes. To test for doneness of marmalade: Drop a spoonful on frozen plate. If marmalade has a slight film when pushed with a finger, it's done. If it spreads out and thins immediately, continue cooking, and test again after a few minutes. Transfer marmalade to airtight containers, cover, and let cool at room temperature. Refrigerate overnight before serving. (Marmalade will keep, covered and refrigerated, for up to 1 month.)