hello. i'm a pharmacist, so i have access to some cool references/resources. there doesn't seem to be anything in watermelon that would act as a diuretic, so any extra peeing would just be from extra water intake. watermelon is a perfectly healthy snack, even in relative excess.
as for watermelons being bred for added sweetness, it is a possibility. there are many different cultivars of melon. it looks like someone has made a new low cal watermelon that they are testing to see if consumers will like it. it might be years before we actually see it in stores though. hope this helps!
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