To the best of my knowledge this has not been done yet.
pumpkins come in 2 types moschata, (winter squash family )
and pepo ( summer squash family )it is hard to cross between these families (almost impossible without gene spicing) therefore my own planned crossbreed of the "big max squashkin " and the "blue hubbard squash" to get "BLUE MAX SQUASHKINS" ,the name alone should get sales in German neighborhoods, has yet to work . you can cross some squash with some gourds and some pumpkins so long as you dont go out of your family . pumpkins x watermellons has yet to be done . You could be the first. best wishes.

I dont know about the whole watermelons and pumpkins thing but i am trying to breed a mix of cantaloupe and watermelon to see if it will work for science fair and i would really like to know if i am just goin to waste a lot of time

This also has not been done yet. musk melon , casabas ,honey dews, and the Armenian "guta" cuke which is really a melon can all cross breed , . there are rare instances of melons crossing with regular cukes . but it is rare. summer squash can not even cross with winter squash . but keep trying you might be the first. Just do not pin your hopes on it .