What is the weight, in ounces or grams of 6 cups watermelon?

Apologies that we don't have this for you, but the inexactness doesn't really hurt this recipe (watermelon varies so much in sweetness anyway, so you may want to tweak the sweetness and dilution of the final tonic to taste anyway). If you don't have access to cup measures, the USDA ingredient database is extremely useful—here's the page for 6 cups of diced watermelon, which they say is about 912 grams: https://ndb.nal.usda.gov/ndb/foods/show/09326?man=&lfacet=&count=&max=25&qlookup=watermelon&offset=&sort=default&format=Abridged&reportfmt=other&rptfrm=&ndbno=&nutrient1=&nutrient2=&nutrient3=&subset=&totCount=&measureby=&Qv=1&Q326501=1&Q326502=6&Q326503=1&Q326504=1&Q326505=10.0&Q326506=1&Qv=1&Q326501=1&Q326502=1&Q326503=1&Q326504=1&Q326505=10.0&Q326506=1

As bpin points out, there's no exact answer; the recipe calls for "coarsely chopped", so you're measuring a lot of air- that will also depend on the size and shape of your measuring cup. A cup of water weighs eight ounces and a watermelon is almost all water, so you can get some sort of clue from that- I'd guess a cup of watermelon at about 6 oz., but your guess is as good as mine.

It all depends on how it is cut up and chopped.