I ended up with probably 2 Qts of watermelon "juice" I had used an ice cream scoop to remove the fruit it was the easiest way to do it. I just blended it to make sure I had maximum flavor.
Then there was the issue of pasteurizing the fruit. I dosed it with campden tabs to not boil off any aromatics and preserve the flavor. I had added it after a week in a secondary carboy, so it was technically a 3 stage fermentation, with 2 weeks on the fruit.
The sugars in watermelon ferment in weird ways, mine had actually started back up after adding the fruit.
One more word of caution, watermelon is a delicate flavor, it's difficult to get it to stick, I would recommend at least 1 whole watermelon, and if I were doing it again, I'd use two.
I think cutting back on the batch size may help, but I found the 2 Qts of juice insured I bottled 5 gallons, and lost nothing to the sediment at the bottom.