Do you have any yellow or dead leaves, or yellow/dead spots on the leaves? My melons got powdery mildew from the hot and dry weather; that particular strain of mildew likes it hot and dry.

Nope, no yellow or dead leaves, just leaves that simply don't seem big enough and that have now started to curl. On the other hand, they don't seem quite as green as I might expect...

It's very normal for watermelon leaves to wilt and curl during the day, they should relax at night. Probably nothing to worry about. They are heavy feeders though, so you am want to fertilize them. Are you growing them in the ground? Some pics would help too.