Q.Watermelon disease
What is the insects on the tomatoes as it was the same on the watermelon?

It is difficult to determine what disease or pests your Watermelon plants have suffered from, as it appears the plant has died off.
Earlier signs on the leaves or stems or fruit itself can help you make a more clear determination.
These links will help you.
https://www.gardeningknowhow.com/edible/fruits/watermelon/watermelon-disease-control.htm
https://www.gardeningknowhow.com/edible/fruits/watermelon/yellow-brown-watermelon-leaves.htm