Your seeds are almost certainly cross pollinated. All watermelon varieties are same species (Citrullus lanatus) and different varieties will/do cross pollinate each other. You are correct that if you plant the seeds you saved you won't know what kind of fruit you're going to get.

Rodney

It doesn't matter how close or far apart they were in your garden. Bees can take the pollen to flowers of the same species up to 1/2 mile away.

Wow....that's what I was afraid of. SO this big bag of seeds I so carefully cleaned and dried and saved are worthless....unless I want to roll the dice. This makes me wonder about all watermelon seeds, though. I mean, does each seed company really grow all the different varieties of watermelon in complete isolation, as much as 1/2 mile from any other watermelon? The company I get my seeds from has about 40 different kinds of watermelons. That's 40 isolated growing spots! How do they assure/maintain a varieties purity? And for heirloom varieties that claim to be a hundred years old or more, how do we know they were always grown in isolation? I am sure I'm the first one to ever wonder about this, so if there is some obvious answer please forgive me, but I'd love to know it. Just curious.
Thanks for the responses!
kevin

Mixed yes, worthless no. Sometimes the mutts are better than the purebreds. Most of the seed venors buy thier seeds from growers, who may or may not use isolation. Bagging flowers and hand pollinating is a viable option for watermelon and other fruits with multiple seeds.

I watched a video on how to do it once. Some female flowers are picked off, so there are fewer to deal with, then the remaining flowers are covered before they open. On the right day (the day the flower opens) hand pollination is done with special care not to let any bees get into the act. Then the flower is kept covered for a while longer.

For fun, do you have some space to plant some of your mutt seeds? Seems worth the experiment.

Thanks for the answers. I do have enough space to try my "mutts" so I may do that, though after reading these answers I reordered some new Orangeglo seeds. One thing about trying my mutts....how awful would it be if I turned out to have some incredible new variety and would never be sure how to recreate it! I had a total of 6 varieties of watermelons all together so there is no telling which one may have pollinated the melon I got these seeds from!
I really do appreciate you all explaining the pollination/offspring thing even though it was probably a very elementary question to most of you. I must say I really enjoy growing several different species of watermelons and cantaloupes so I guess I'll just be buying new seeds each year! Thanks again.

Seeds do last more than one year. (if you had any left over) What I do is I just plant more seeds as the years go by and as germination rates go down.

Then again, in 1/2 acre, maybe you used all your seeds?
So here's a dumb question for you: are you selling watermelons?