Cross pollinating watermelons and pumpkins
Is it possible to cross pollinate watermelons and pumpkins?
Certified GKH Gardening Expert
It's possible, because they're both in the cucurbitaceae family (pumpkins and melons). Keep in mind you won't see the effects on this year's fruit (if you're trying to cross-pollinate on purpose), since it only affects the seeds from this year's plant (and so you'll only see the result in next year's fruit if you plant those seeds).
This article talks a bit more about cross-pollination: