If weather cooperates it takes 75-100 days to get watermelons. If you get early variety and leave just the first bunch of fruits on your vine, you will probably get watermelon in late September. What is your weather like then? If it is still hot, I'd go for it. :)

You can always try saving hybrid seed. It may not come true to the parent, but in some cases it may. After all that is how breeding new varieties is/was done. Cross breeding and then selecting for desired characteristics over several generations. There also are some varieties which are labeled as hybrid but are not. I've always suspected that it was done to keep people from saving seed. Anyway if you have the watermelons growing now, might as well save some seed and give them a try next year. It may or may not work out, but you have little to loose other than some wasted space in next years garden.

It's hard to venture a guess, without seeing a photo. If it is some type of fungas, the Immunox should help. Melons need a good bit of spacing between plants, and the yellowing may be due to the plants competing against on another. Kind of like survival of the fittest.

Thanks for all the suggestions. Couldn't find the information I was looking for to cure this organicly, so I did finally get a fungicide / insecticide combination spray. It wasn't organic, although the label states that it's safe to use on fruit and vegetable plants. (I'll have to wash the fruit really well before cutting into it.) The stuff smells horrible, but I think it may have done the trick. There is still leaf damage, but I have a new shoot starting from the main vine and the watermelon fruit doesn't seem to be as soft anymore. Here's for hoping. Next year I'm going to use a fungicide before the plant starts to flower and hopefully that'll prevent this from happening again. Thanks everyone for your help!