Cut, puree & strain watermelon to produce 1 qt of juice.
Set the juice in a large saucepan over high heat to bring to a full boil.
While juice is warming, add lemon juice and calcium water; stir to combine.
Also while juice is coming is coming to a full boil, combine the sugar and the pectin in a small bowl.
Once the juice is at a full boil, slowly pour in the sugar-pectin mix, stirring vigorously to dissolve.
Once everything is dissolved, and it is back to a boil, remove the pan from the heat and let it set just a bit, stirring with a spoon periodically to get the bubbles out. Skim off the foam, if you don't want it in your finished product.
Ladle into jars, leaving 1/4" headspace, then process in a hot water bath for 10 minutes. Then let it rest an additional 5 minutes before removing the lid.
Set the jars, without tilting them, on a kitchen towel on the counter.
Let rest 12-24 hours before testing the seal.