Here's a link that might be interesting for you

I actually managed to grow some watermelons one year after a complete failure the previous year.
I had trouble getting fruits to 'set'. My neighbour taught me how to give nature a helping hand by using a soft cosmetic brush to 'hand pollinate'. Had a bumper crop that year that everyone said was the tastiest watermelons they'd ever had.

Here's the method to use. You'll need to do this early in the mornings. Before heading off to work.

The watermelon has female and male flowers.
The female flower has a small bulb just underneath its flower. The male flower doesn't have anything.
Using the soft brush gently pick up some pollen from inside a male flower. You'll actually see the small yellow grains of pollen on the brush hairs.
You now need to very gently transfer that pollen to the inside of the female flower.
The female flower will actually closes its petals after a few hours to make sure it's no longer available for pollination.
With a day or two the female bulb will have grown a little. Means success.

If the female flower hasn't been pollinated within 24 hours of opening, it will just shrivel and drop off.
Unfortunately the male and female flowers don't come at the same time. The male ones come first. So the more plants you have to flower the
the better the chance of having male and female flowers available together for pollination.