We can't access your images. Is your watermelon planted in the ground or a container?

Anyway, what you've described (looks just one one day and wilted the next) sounds just like the work of squash vine borer. Look along the stem to see if you can find the tell-tale holes. You should examine the lower part of the stem for problems, regardless of whether your looking for the squash vine borer or not.

Call your local county extension office to see if the timing is right for this pest to have emerged. It might be a possibility that far south.

Sorry, I'll try to upload the pics another way... The plant is in the ground, and I checked the stems, and they actually look ok. I thought perhaps the roots were getting too hot, because it has been pretty hot here, so I put some fresh mulch down around it... we'll see if that helps.

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