pRoFiT
Can't buy it?, build it!

Kill it now before its too late!

Well what ever it is... it is not surviving Arizona's winter.
It has stopped growing and the leaves are turning yellow.
I cracked one open and it smells exactly like a cucumber.
Weird...
I just came across this thread. I would NEVER had said watermelon as I've never seen one grow on a squash plant. (Which those leaves more closely resemble to me.)
But it is in fact a melon of SOME kind. I'd bet a dollar it's a Santa Claus type mutant. Give me a minute and I might even recall a better candidate..(Santa Claus usually aren't round.)
However, it's going to be pretty hard to nail it down with such a stressed immature fruit. However, it looks like you could dry out those seeds and plant them out next year and get a better ID.
It's pretty much common for almost all melons to smell like cucumbers when immature.
Those are DEFINITELY NOT watermelon seeds! Classic cantalope, honeydew, Isreal melon seeds.