If you choose to use the whole fruit method, where whole citrus fruit are boiled for almost two hours until very soft, there's no specific water amount to use: you use as much water as you need to keep the citrus immersed and simmering. You will dump off that water and when it comes time to make marmalade with the boiled fruit, you won't add any water and only add the the juices from the fruit. For the sliced fruit method where sliced citrus fruit are boiled for one to two hours to soften, generally for around 600 grams of citrus, use 6 cups (1.5 L) and for 800 grams of citrus, use 8 cups (2 L) of water. The more citrus fruit you have, the more water you will need. Just remember that once the citrus peel is sufficiently soft, you will likely be left with 1/3–1/2 of the volume of water.