Watermelons have separate

Watermelons have separate male and female flowers on the SAME plant so they don't need another plant to pollinate. What they need are the pollinators—the bees. Watermelons do cross-pollinate with other watermelon varities. If you do decide to encourage cross-pollination, then I imagine you would want flowers opening around the same time. The female watermelon flowers do not open for very long and this is a morning event. You can always hand pollinate by plucking off a healthy male flower, removing its petals, and wiping the pollen on its stamen on to the center of the female flower. Note that you will not have a hybrid in this first generation. You need to save the seeds from the crossed melon and plant them next season to see what you get!